Small compliance issues in the new String polyfills

[anba]

The new String polyfills are not completely spec compliant for certain edge cases.

Two issues in String.prototype.startsWith:

• RegExp checks needs to be performed after ToString(this Argument)
• arguments[1] needs to be arguments.length > 1 ? arguments[1] : undefined b/c of inherited properties from Object.prototype

One issue in String.prototype.endsWith:

• RegExp checks needs to be performed after ToString(this Argument)

One issue in String.prototype.contains:

• arguments[1] needs to be arguments.length > 1 ? arguments[1] : undefined b/c of inherited properties from Object.prototype

Three issues in String.raw:

• ToObject(raw) must be performed only once
• ToUint32() needs to be replaced with ToLength()
• substitutions is a rest-array, that means it inherits properties from Array.prototype

---

Simple test cases for each issue:

String.prototype.startsWith.call({toString(){ throw 0 }}, /./);

Expected: throws 0 Actual: throws TypeError

Object.prototype[1] = 2;
"ab".startsWith("ab");

Expected: returns true Actual: returns false

String.prototype.endsWith.call({toString(){ throw 0 }}, /./);

Expected: throws 0 Actual: throws TypeError

Object.prototype[1] = 2;
"ab".contains("ab")

Expected: returns true Actual: returns false

Object.defineProperty(Boolean.prototype, "length", {
  get() {
    "use strict";
    if (typeof this == 'boolean') throw new Error("missing ToObject");
    return 0;
  }, configurable: true
});
String.raw({raw:true});

Expected: "" Actual: throws Error("missing ToObject")

String.raw({raw: {length: -1}})

Expected: returns "" Actual: most likely causes an out-of-memory error in browsers...

Array.prototype[0] = "!";
String.raw({raw:"ab".split("")});

Expected: "a!b" Actual: "aundefinedb"

[arv]

@mathiasbynens FYI

[arv]

Thanks for the detailed report.

[mathiasbynens]

@anba Awesome feedback, as usual :) I’ll get these fixed. Thanks!

[anba]

You're welcome! And since I'm re-using parts of the traceur test suite for my own project, it's only fair to file bug reports every now and then. ^^

[mathiasbynens]

@anba Actually, the tests for the first three bugs you reported were taken from http://mths.be/startswith, http://mths.be/endswith, and http://mths.be/contains. I just happened to contribute them to Traceur :)

I’ve fixed the issues you mentioned in those three projects and will be submitting patches to Traceur as soon as possible.

FWIW, in environments where Object.prototype.toString or String.prototype.indexOf is overridden, most of these polyfills will fail too. Not sure if that’s something we need to guard against. @arv, any thoughts?

[arv]

@mathiasbynens I was considering creating a module that captures these as early as possible.