Compiling Es6 Arrow Functions To Es5 Using Babel.js

While looking into ES6 arrow functions' documentation on Mozilla documentation, I got to know that Arrow functions applies all the rules of strict mode, except one as described in

Solution 1:

tl;dr: Babel assumes every file is a module. Modules are strict by default and their this value is undefined.

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This is covered in the Babel FAQ:

Babel assumes that all input code is an ES2015 module. ES2015 modules are implicitly strict mode so this means that top-level this is not window in the browser nor is it exports in node.

If you don't want this behaviour then you have the option of disabling the strict transformer:

$ babel --blacklist strict script.jsrequire("babel").transform("code", { blacklist: ["strict"] });

PLEASE NOTE: If you do this you're willingly deviating from the spec and this may cause future interop issues.

See the strict transformer docs for more info.

Solution 2:

You are correct in principle, as described on MDN. However, Babel always places a 'use strict' at the root scope. In effect you compile the following:

'use strict';
varf = () => { 'use strict'; returnthis};

In that case strict rules do apply. See the compiled example here. Babel even optimizes away the top-level this as it is guaranteed to be undefined.