Why does this throw an error in Node.js with --harmony?

Question

In Chrome Canary and Node.js 0.12.3, the following code prints p.

  'use strict';
  let o = {
    name: 'o',
    foo: function() {
      ['1'].map(function() {
           console.log(this.name);
      }.bind(this));
    },
  };

  let p = { name: 'p' };

  o.foo.call(p); // p

In Chrome Canary the following code also prints p. But why does it throw a TypeError in Node.js 0.12.3 with the --harmony flag?

  'use strict';
  let o = {
    name: 'o',
    foo: function() {
      ['1'].map(() => {
           console.log(this.name);
      });
    },
  };

  let p = { name: 'p' };

  o.foo.call(p); // p in Chrome, TypeError in Node.js with --harmony

Put another way, why is this undefined when the second code snippet is run in Node.js?

Arrow functions are broken in node/io right now https://github.com/nodejs/io.js/issues/735 — Explosion Pills, Jun 22 '15 at 19:55
This is almost certainly due to the fact that Canary is using a more recent version of v8 than Node is (since Canary is intentionally bleeding-edge, while Node tries to keep it stable). — apsillers, Jun 22 '15 at 20:00

score 2 · Accepted Answer · answered Jun 26 '15 at 13:08

This is simply due to a bug in the version of the V8 engine that iojs and node use. Chrome Canary uses an unstable version of V8 that has addressed this issue. When this fixed is rolled out to a stable version of V8, node/iojs should work the same way.

For now, you can use a tool like babel to transpile your code. Using babel with no flags on your code transpiles to:

function foo() {
  var _this = this;

  ['1'].map(function () {
    console.log(_this.name);
  });
}

Which does indeed print p.

Why does this throw an error in Node.js with --harmony?

1 Answers1