强制进行不正确的比较以在JavaScript中引发警告/错误

Question

I had JavaScript code similar to the following:

var data = {"d": 12345};
for(var i = 0; i < data.length; ++i) {
    console.log(i);
}

I know that this for loop will not execute any cycles because data.length will be 'undefined' and

0 < 'undefined' //false (I think this is false for any int?)

Anyways, while I fixed my code (in my code, data contains an array but I was accidentally referencing data.length instead of data.myarr.length) I'm hoping to avoid such problems in the future - took me quite some time to find this bug.

I have been reading up on strict mode in JavaScript but after messing around with JSFiddle I couldn't get strict mode to throw the above as an error. I tried both making the entire script strict, then, thinking maybe there was a conflict, tried it within its own method and still got nothing.

Is there a way to force "nonsensical" comparisons such as int < "undefined" to throw an error (or even a warning)? Did I miss something with strict mode? Or is the language design supposed to allow such things?

Answer 1

There is nothing within Javascript itself that will pick this one up.

I haven't kept track of the latest developments in JS Lint , but I don't know of anything there that would help.

On the other hand, TypeScript handles this situation very well.

Just as head's up -- this isn't something that can change in Javascript without breaking existing code. There are enough array like objects that have a both a length property and numericly named property names that one iterates over.

• The arguments keyword
• Node lists

Answer 2

AFAIK, there's no real way to achieve what you're after. since int < undefined is a perfectly fine comparison.

One thing you could do, for this specific issue, is use Array.forEach ( MDN ). This would throw an error since data.forEach is undefined and attempting to call that as a function will fail ( unless you added a "forEach" key on data with a function value, of course ):

var data = {"d": 12345};
data.forEach(function(item, index, array){
    console.log(index);
});

Note, that IE < 9 does not have Array.forEach, so you would need to add it to Array's prototype. See more here, including how to do so, at Array.prototype.forEach() on MDN .

Answer 3

In ES6, you could use an iterator via for...of , which would result in an error since non-array objects are not iterable:

for (var x of data) { }

You will have make other arrangements if you also want the index.