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.

...

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.

...

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.

...

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 (including the default) 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.

...

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 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.

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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.

...

// 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:
            throw Error("Invalid Operation Mode")
            break;
    	default:
    		throw Error("Unrecognized Error Code")
    }
} catch (error) {
	var errorString = "Could not turn on motor: "
    errorString += error.message;
    notification.send(errorString)
}

Built-In Functions for Script Importing and Execution

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

Examples forsystem.importScript and system.runScript

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

Running Scripts in Order

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

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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 begins running the script on another thread.

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Note that with 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:

...

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.

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