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Scripts in event sheets

Ashley's avatar
Medal
Construct Team Founder
Published 28 May, 2019
878 words
~4-6 mins

A great way to get started with JavaScript is writing snippets of JavaScript in the event sheet. You can write some JavaScript code to run in the place of an action or an event block.

Scripts in actions

JavaScript code can be added as an action. The code runs whenever the action is run, i.e. after all the event's conditions are met, and all previous actions have run. To add some code as an action, use the Add... menu to the right of the Add action link, and select Add script. Alternatively you can use the J keyboard shortcut when an action is selected.

A code editor will appear for you to enter the JavaScript code to run.

Scripts in blocks

Alternatively JavaScript code can be added as a block, in the same place as other event blocks appear. The code runs whenever an event block in that place would be run. You can add a script block using any of the following methods:

  1. Right-click in the margin of an existing event block and select AddAdd script
  2. Click the Add... menu at the end of a group, or the end of the event sheet, and select Add script
  3. Use the J keyboard shortcut when an event block is selected (note if an action is selected, a script action will be inserted instead)

Remember that the event sheet runs all events in top-to-bottom order every tick, so a script in a block at the top level of an event sheet is run every tick (as if it was an action in an Every tick event). Often it is more useful to use script blocks as sub-events, which only run if their parent event block was true.

Using 'await'

Code written in either a script action or script block is actually run inside an async function. This means your code can use the await keyword. For example you can await the result of a fetch over the network.

Note that the rest of the event sheet will continue to run while await waits for its result. In other words, await pauses the JavaScript execution and any actions after the code will immediately run. Then once the awaited promise resolves, any code after the await will then run - which will be after the following actions have run. You can change this using the System action Wait for previous actions to complete: this will wait for any code blocks before it to finish before running the following actions.

Using the runtime interface

All scripts in the event sheet can access a special runtime variable which refers to the runtime script interface. This provides functions and properties that lets your code access and control Construct's runtime. This also includes ways to closely integrate code and events, such as iterating the instances picked by the current event.

A very simple example is shown below, which can be used to show a dialog box with the project name in it.

alert(runtime.projectName);

Accessing local variables

A useful way to pass values between scripts and the event sheet is to use local variables in the event sheet. These can be accessed by both script actions and script blocks using the variable name localVars. This is set to an object with a property for each local variable in scope. The available local variables are the same as are available to a Set value action in the same place. This includes any parameters for event sheet functions.

For example a script in an event group with a local variable named temp can access the local variable using localVars.temp. A useful pattern is to use an action to set a local variable to an expression, and then read from it in a following script action. Alternatively a script could calculate a value and assign it to a local variable, to subsequently be used in the event sheet. It could also be used both ways at once, both reading the variable and then assigning it.

Note that localVars excludes global variables, which are available via runtime.globalVars instead. localVars is also unique to scripts in the event sheet - script files cannot access it, because they do not have a scope.

Errors

Any exceptions, or rejected promises, arising from a script in an event sheet will be caught by the Construct engine and logged to the console with information about where the error came from. This means unhandled exceptions or rejections will not crash the game (since browsers stop running scripts if they encounter an unhandled error). However you should keep an eye on the browser console to check for any unexpected errors. For more information see the section on debugging script.