Many use the MDC fallback implementations (eg. for indexOf). They're generally rigorously standards-compliant, even to the extent of explicitly checking the types of all the arguments.
Unfortunately whilst it is clear that the authors regard this code as trivial and freely-usable, there doesn't seem to be an explicit licence-grant to put this in writing. The wiki as a whole is CC Attribution-ShareAlike, if that's an acceptable licence (though CC isn't designed for code as such).
js-methods looks OK in general, but is not as standards-compliant around the edges of how the functions are supposed to be (eg. undefined list items, functions that mutate the list). It's also full of other random non-standard methods, including some questionable ones like the dodgy stripTags and the incomplete UTF-8 codec (which is also a bit unnecessary given the unescape(encodeURIComponent) trick).
For what it's worth, here's what I use (which I hereby release into the public domain, if it can be said to be copyrightable at all). It's a bit shorter than the MDC versions as it doesn't attempt to type-sniff that you haven't done something silly like pass non-function callbacks or non-integer indexes, but apart from that it attempts to be standards-compliant. (Let me know if I've missed anything. ;-))
'use strict';
// Add ECMA262-5 method binding if not supported natively
//
if (!('bind' in Function.prototype)) {
Function.prototype.bind= function(owner) {
var that= this;
if (arguments.length<=1) {
return function() {
return that.apply(owner, arguments);
};
} else {
var args= Array.prototype.slice.call(arguments, 1);
return function() {
return that.apply(owner, arguments.length===0? args : args.concat(Array.prototype.slice.call(arguments)));
};
}
};
}
// Add ECMA262-5 string trim if not supported natively
//
if (!('trim' in String.prototype)) {
String.prototype.trim= function() {
return this.replace(/^\s+/, '').replace(/\s+$/, '');
};
}
// Add ECMA262-5 Array methods if not supported natively
//
if (!('indexOf' in Array.prototype)) {
Array.prototype.indexOf= function(find, i /*opt*/) {
if (i===undefined) i= 0;
if (i<0) i+= this.length;
if (i<0) i= 0;
for (var n= this.length; i<n; i++)
if (i in this && this[i]===find)
return i;
return -1;
};
}
if (!('lastIndexOf' in Array.prototype)) {
Array.prototype.lastIndexOf= function(find, i /*opt*/) {
if (i===undefined) i= this.length-1;
if (i<0) i+= this.length;
if (i>this.length-1) i= this.length-1;
for (i++; i-->0;) /* i++ because from-argument is sadly inclusive */
if (i in this && this[i]===find)
return i;
return -1;
};
}
if (!('forEach' in Array.prototype)) {
Array.prototype.forEach= function(action, that /*opt*/) {
for (var i= 0, n= this.length; i<n; i++)
if (i in this)
action.call(that, this[i], i, this);
};
}
if (!('map' in Array.prototype)) {
Array.prototype.map= function(mapper, that /*opt*/) {
var other= new Array(this.length);
for (var i= 0, n= this.length; i<n; i++)
if (i in this)
other[i]= mapper.call(that, this[i], i, this);
return other;
};
}
if (!('filter' in Array.prototype)) {
Array.prototype.filter= function(filter, that /*opt*/) {
var other= [], v;
for (var i=0, n= this.length; i<n; i++)
if (i in this && filter.call(that, v= this[i], i, this))
other.push(v);
return other;
};
}
if (!('every' in Array.prototype)) {
Array.prototype.every= function(tester, that /*opt*/) {
for (var i= 0, n= this.length; i<n; i++)
if (i in this && !tester.call(that, this[i], i, this))
return false;
return true;
};
}
if (!('some' in Array.prototype)) {
Array.prototype.some= function(tester, that /*opt*/) {
for (var i= 0, n= this.length; i<n; i++)
if (i in this && tester.call(that, this[i], i, this))
return true;
return false;
};
}
Other ECMA262-5 methods not implemented here include Array reduce/reduceRight, the JSON ones and the few new Object methods that can be reliably implemented as JS functions.