The following table lists keywords and statements that can be used in a script for better control flow.

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Name

...

Description

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if…else

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The if statement executes some specified instructions if the condition is true. Otherwise, the else statement can be used to execute a different set of instructions. else if can be used to specify an alternative condition with different instructions. Once an if or else if statement is executed, any following else if statements will not be executed.

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while

...

A while loop will repeatedly execute until the condition expression becomes false.

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do…while

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The do statement allows you to execute the contents of a while loop at least once before the condition is evaluated. If the condition is true, the loop will repeat as usual until the while condition becomes false.

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for

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A for loop repeats until a specified condition becomes false. Below is the standard syntax used by a for loop:

for (<initialExpression>; <conditionExpression>; <incrementExpression>) {
    // instruction code
}

A for loop can also be used to iterate over arrays and objects, as explained below.

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break

...

Terminates a switch statement, a while loop, or a for loop completely. The break statement prevents the execution of the following cases in a switch statement. Adding a break statement to each case is good practice.

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continue

...

The continue statement terminates the execution of the current iteration in a loop and then executes the next iteration. Unlike break, this statement does not terminate the loop completely.

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switch…case

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The switch statement lets you compare an input value to the value of each case. If matched, the instruction code for that case is executed. Otherwise, the default instruction code is executed. Each case statement should usually be followed by a break statement.

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default

...

The default statement specified what should happen if a switch statement does not match any of the cases. A break statement is not required.

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function

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A function is a block of code designed to perform a particular task. Functions can take in a set of arguments and can return a value. Functions are one of the primary methods for organizing and re-using code.

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return

...

Ends the function execution, and the function will return the specified value to the caller.

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arguments

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Can be used within a function to get a list of values that were passed into the function call.

Examples for if…else and else if:

In this example, a notification saying “boolTag is true” will be displayed if the tag’s value is true.

if (tag.read("boolTag")) notification.send("boolTag is true.");

You can execute multiple lines of code after an if statement by enclosing the code in curly braces:

if (tag.read("boolTag")) {
    notification.send("boolTag is true.");
    thread.sleep(1);
}

In the example below, if tag “A” equals 1, assign 100 to tag “B”. If “A” equals 2, assign 200 to “B”. Otherwise, assign 500 to “B”.

var value = tag.read("A");
if (value == 1) {
    tag.write("B", 100);
} else if (value == 2) {
    tag.write("B", 200);
} else {
    tag.write("B", 500);
}

Note that the if and else if conditions must be enclosed in parentheses.

...

The tag "A" will be incremented from 0 to 100 over the course of one second:

var val = 0;
while (val <= 100) {
	tag.write("A", val);
	val = val + 1;
	thread.msleep(10);
}

Example for do…while:

The tag “boolTag” will be toggled at least once. If the tag “doLoop” is true after one second, “boolTag” will continue to toggle once per second. Once “doLoop” becomes false, the toggling will stop and the script will end.

do {
    tag.write("boolTag", !tag.read("boolTag"));
    thread.sleep(1);
} while (tag.read("doLoop"));

Examples for for

A for loop lets you provide 3 expressions that define the behavior of the loop.

• The first expression will be run once before the loop first executes. Typically, this is used to initialize a counter variable like i or j.

• The second expression is a condition that will be checked before each execution of the loop. If the condition is false, the loop will not execute, and the for loop will end. Typically, this is used to check if the counter variable has reached some value.

• The third expression is run at the end of each execution of the loop. Typically, this is used to increment the counter variable.

In this example, the initial value of i is 0. When i is less than or equal to 100, i will increment by 1 and store the result in the tag The following table lists keywords and statements that can be used in a script for better control flow.

for (<initialExpression>; <conditionExpression>; <incrementExpression>) {
    // instruction code
}

Examples for if…else and else if:

In this example, a notification saying “boolTag is true” will be displayed if the tag’s value is true.

if (tag.read("boolTag")) notification.send("boolTag is true.");

You can execute multiple lines of code after an if statement by enclosing the code in curly braces:

if (tag.read("boolTag")) {
    notification.send("boolTag is true.");
    thread.sleep(1);
}

In the example below, if tag “A” equals 1, assign 100 to tag “B”. If “A” equals 2, assign 200 to “B”. Otherwise, assign 500 to “B”.

var value = tag.read("A");
if (value == 1) {
    tag.write("B", 100);
} else if (value == 2) {
    tag.write("B", 200);
} else {
    tag.write("B", 500);
}

Note that the if and else if conditions must be enclosed in parentheses.

Examples forwhile:

The tag "A" will be incremented from 0 to 100 over the course of one second:

var val = 0;
while (val <= 100) {
	tag.write("A", val);
	val = val + 1;
	thread.msleep(10);
}

Example for do…while:

The tag “boolTag” will be toggled at least once. If the tag “doLoop” is true after one second, “boolTag” will continue to toggle once per second. Once “doLoop” becomes false, the toggling will stop and the script will end.

do {
    tag.write("boolTag", !tag.read("boolTag"));
    thread.sleep(1);
} while (tag.read("doLoop"));

Examples for for

A for loop lets you provide 3 expressions that define the behavior of the loop.

• The first expression will be run once before the loop first executes. Typically, this is used to initialize a counter variable like i or j.

• The second expression is a condition that will be checked before each execution of the loop. If the condition is false, the loop will not execute, and the for loop will end. Typically, this is used to check if the counter variable has reached some value.

• The third expression is run at the end of each execution of the loop. Typically, this is used to increment the counter variable.

In this example, the initial value of i is 0. When i is less than or equal to 100, i will increment by 1 and store the result in the tag “FOR_VAL” every iteration until i equals 100. Once i equals 100, the for loop will end.

for (var i = 0; i <= 100; i++) {
	tag.write("FOR_VAL", i)
	thread.msleep(100)
}

Looping Over Arrays

A for loop can also be used to iterate through arrays using the array index.

// Define an array
var myArray = [
    "val1",
    "val2",
    "val3"
]

// Get the indices
for (var index in myArray) {
	notification.send(index);
} // 0, 1, 2

// Get the values
for (var index in myArray) {
	var value = myArray[index];
    notification.send(value);
} // val1, val2, val3

Looping Over Objects

A for loop can also be used to iterate through objects using keys.

// Define an object
var myObject = {
  "key1": "val1",
  "key2": "val2",
  "key3": "val3"
}

// Get the keys
for (var key in myObject) {
    notification.send(key);
} // key1, key2, key3

// Get the values
for (var key in myObject) {
    var value = myObject[key];
    notification.send(value);
} // val1, val2, val3

Example forcontinue

The continue keyword can be used in a while or for loop to move to the next iteration without breaking out of the loop. This can be used to skip specific items.

...

for (var i = 0; i < 10; i++) {
    if (i == 5) {
        continue;
    }
    notification.send(i);
} // 0, 1, 2, 3, 4, 6, 7, 8, 9

Examples for switch…case, break, and default

In this example, when the script executes, the tag "REPLY" will be assigned the string value "Hello, <name>" by default. If the name entered is empty, the “REPLY” tag will be set to "Could not load name." instead.

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The break statement is used to break out of the switch statement and prevent any other cases from executing. If break is not used, then multiple cases may be executed. This can lead to unexpected behavior, so in most cases, each case should have a break statement.

Examples for function,return, and arguments

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function multiply(a, b) {
	return a * b;
}
var x = multiply(4, 3); // x is now equal to 12

Optional Arguments

Functions do not need to have any arguments. For example, here is a function that outputs a message, waits for 3 seconds, then exits the project.

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In the example below, the function multiplyDefault returns a * b if both a and b are specified. If b is not specified, the function instead returns a * 10.

function multiplyDefault(a, b) {
    if (b === undefined) {
        b = 10;
    }
	return a * b;
}
multiplyDefault(2, 3); // 6
multiplyDefault(2); // 20

The arguments keyword

There is also a special keyword arguments, which gives an array containing the values passed into the function call. This allows functions to use a variable number of arguments.

function returnFirstArg() {
	return arguments[0];
}
returnFirstExtraArg("a", "b", "c"); // "a"

You can also use arguments in a function definition, in which case arguments will only include values not covered by the named arguments. This is useful when you know that the first few arguments are required, but any additional arguments are optional.

function returnFirstExtraArg(x, y, arguments) {
	return arguments[0];
}
returnFirstExtraArg("a", "b", "c"); // "c"

Examples for try…catch and throw

A try…catch statement can be used to gracefully handle situations that would normally cause the script to terminate. If the code in the try block fails, this error is returned to the catch block, which either makes use of or ignores the error.

In the catch statement, you must give a name for the error from the try block. It is common practice to use the name error.

The throw statement can be used to manually create an error. This is mainly useful when used within a try block, since the custom Error object can then be used by the catch block.

However, throw can also be used outside of a try block, in which case the entire script will terminate with an error. This can be useful for scripts that are called via system.importScript, or when you need to prevent the rest of the script from executing.

Checking Whether a Tag is Loaded or Not

When a project first starts, some tags may not be loaded yet, in which case tag.read or tag.write will throw an error. Remote tags may take a few seconds to load, depending on the polling rate, communication method, and the size of the project.

The example script below would run on project startup and wait for a remote tag to load (become readable). Once we know that the tag can be read, we can run the rest of our startup actions (which might make use of the tag).

// Inside main startup script "Startup1"
var loading = true;
var waited = 0;
while (loading) {
	try {
		tag.read("remoteTag");
		loading = false;
	} catch (error) {
		if (waited > 10000) {
			throw Error('Could not load tag "remoteTag" after 10 seconds. Please check the connection and restart the project.');
		}
		thread.msleep(100);
		waited += 100;
	}
}
// Now do any other startup actions

Checking an Error Code From a Tag

The example below checks the value of an error code tag “errorCode” before attempting to turn a motor on using the tag “motorOn”. Depending on the error encountered (if any), the script will display a different notification. If there is a problem with tag.read or tag.write, this will also be handled using the notification instead of terminating the script.

try {
    var errorCode = tag.read("errorCode");
    switch (errorCode) {
    	case 0:
    		tag.write("motorOn", true);
    		break;
    	case 1:
    		throw Error("Power Disconnected");
    		break;
    	case 2:
    		throw Error("Mechanical Failure")
    		break;
        case 3:
           b = 10;
 throw Error("Invalid Operation Mode")
  } 	return a * b; } multiplyDefault(2, 3); // 6 multiplyDefault(2)break;
// 20

The arguments keyword

There is also a special keyword arguments, which gives an array containing the values passed into the function call. This allows functions to use a variable number of arguments.

function returnFirstArg() { 	return arguments[0];
}
returnFirstExtraArg("a", "b", "c"); // "a"

You can also use arguments in a function definition, in which case arguments will only include values not covered by the named arguments. This is useful when you know that the first few arguments are required, but any additional arguments are optional.

function returnFirstExtraArg(x, y, argumentsdefault:
    		throw Error("Unrecognized Error Code")
    }
} catch (error) {
	return arguments[0];
}
returnFirstExtraArg("a", "b", "c"); // "c"var errorString = "Could not turn on motor: "
    errorString += error.message;
    notification.send(errorString)
}

Built-In Functions for Script Importing and Execution

Canvas also provides some built-in functions which allow you to utilize scripts from within other scripts.

Examples forsystem.importScript and system.runScript

Both system.importScript and system.runScript can be used to execute scripts in the project from within another script. The difference is that importScript runs the script in the same thread, and runScript runs the script in a different thread.

Say we have 3 To compare the two functions, consider these scripts “A”, “B”, and “C”:

...

// C
notification.send("In C");

Running Scripts in Order

We can use system.importScript to execute the scripts in order, like so:

...

While the target script is running, the calling script is paused. If there is an error in the target script, the calling script will have an error as well.

Running Scripts Immediately

Scripts can also be run in parallel using system.runScript. This immediately runs begins running the script on another thread.

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With runScript, as soon as the target script begins, the calling script resumes execution.

Note that with this method, runScript you cannot guarantee the order of execution. In this case, the order of steps 1 and 2 may be reversed, as well as steps 4 and 5.

Importing Functions and Variables

system.importScript can be used to import functions and variables from other scripts. In this example, we have a script called "myFunctionScript", which has a function fib that returns the Fibonacci sum:

// myFunctionScript
function fib(n) {
    if (n <= 0) {
        return 0;
    } else if (n == 1) {
        return 01;
    } else if{
(n == 1) {     return fib(n -  return 11) + fib(n - 2);
    }
else {
        return fib(n - 1) + fib(n - 2);
    }
}
}

If we want to use the function fib in a different script, we just need to import "myFunctionScript" first:

system.importScript("myFunctionScript");
notification.send(fib(7)) // 13

This also works with variables. For example, say we have another script called “myVariableScript”:

// myVariableScript
var x = 7;

We could then load the variable x as follows:

system.importScript("myVariableScript");
notification.send(x); // 7

Getting a Return Value From a Script

system.importScript returns the value of the last expression in the target script.

Here is a simple example where the target script returns the value 12. This value is then used in the calling script.

//targetScript
var x = 7;

If we want to use the function fib and the variable x in a different script, we just need to import "myFunctionScript" first:

x + 5;

// Calling Script
var returnValue = system.importScript("myFunctionScripttargetScript");
notification.send(fib(xreturnValue)); // fib(7) -> 1312