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Gepser Hoil
Asked: 2020-03-08 07:36:07 +0800 CST 2020-03-08 07:36:07 +0800 CST

Why is there a syntax error in this arrow function?

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I'm reading about the special cases of funciones flecha (arrow functions) in javascript and came across an example that I couldn't run.

The example is:

[[1,2], [3,4]].map(([a,b]) => a + b);

which if we try to print it to the console should give us this result:

console.log([[1,2], [3,4]].map(([a,b]) => a + b));

//[ 3, 7 ]  <== resultado

The problem is that when trying to execute that line, I get the following error:

console.log([[1,2], [3,4]].map(([a,b]) => a + b));
                            ^

SyntaxError: Unexpected token [
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:374:25)
    at Object.Module._extensions..js (module.js:405:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:430:10)
    at startup (node.js:141:18)
    at node.js:1003:3

I am using strict mode:

'use strict';

I have done the tests with these versions of node (on OSX):

5.4.0
5.0.0
4.3.0
4.0.0

In all cases, the result has been the same, why?

javascript

1 Answers

  1. Best Answer

    The reason it may not work on some platforms or browsers is because the ECMAScript 2015 Destructuring assignment feature that allows the "unset array" syntax is still under development on some platforms:

    Node.js

    In Node.js Destructuring assignment is experimental and disabled by default.

    In order to enable it, run Node with the flag--harmony-destructuring

    Example:

    node --harmony-destructuring script.js

    Chrome

    In Chrome Destructuring is activated by default since version 49 , so the example should run without problems with the latest version of Chrome.

    In previous versions:

    1. navigate tochrome://flags/#enable-javascript-harmony
    2. Enable the feature
    3. Restart the browser

    Edge

    Edge implements it as experimental features. In order to activate support:

    1. navigate toabout:flags
    2. Check the Experimental features --> JavaScript --> Enable experimental JavaScript features
    3. Restart the browser
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