Why putting colon before variable name is a valid code in js?

Question

Example:

let :test = "Hello";
console.log(test);

This code doesn't throw any error. Why?

The code is interpreted as let: test = "Hello"; the : is part of let which is a label here. You can tell that this is not a variable declaration because the console prints the value you assign. If it was a variable declaration it would print undefined. Also have running the code, test exists (not :test) and has the value "Hello". Same effect: foo :bar = "Hello"; — Felix Kling
– Felix Kling, Commented Mar 27, 2019 at 21:20
@FelixKling Thanks for the answer! One more question. Why in this case it throws an error? (without strict mode) const :test = "Hello"; console.log(test); — Nevermind
– Nevermind, Commented Mar 27, 2019 at 21:40
let is not a reserved word in non-strict mode: ecma-international.org/ecma-262/6.0/…, which means it can be used as variable name and other places. E.g. var let = 42; is valid in non-strict mode. — Felix Kling
– Felix Kling, Commented Mar 27, 2019 at 21:43

epascarello · Accepted Answer · 2019-03-27 21:19:33Z

5

If you were in strict mode, it would have thrown an error.

let: // <-- label
  test = "hello

answered Mar 27, 2019 at 21:19

epascarello

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