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I spent hours just to find out that I misspelt the word .length with .lenght. It can run normally with no warning at all. Why...?

I use 'use strict' and run on Node 10.13.0.

Code:

'use strict';

let arr = [1, 2, 3, 4];

for(let i = 0; i < arr.lenght; i++) {

  console.log(arr[i]);

}

Because when you try to get a property that does'nt exist, it returns undefined, and 0 < undefined is false.
Take into account that javascript is not a strongly typed language. You can add new properties by simply giving it a value arr.something=123, even you can set arr.length=7 but it's not a good idea.

let arr = [1, 2, 3, 4];

console.log(arr.lenght) // undefined

console.log(arr.qwerty) // undefined

console.log(arr.lenght < 9999) // false

console.log(arr.lenght > 9999) // false



arr.length = 7 // <-- it's not a good idea

for(let i = 0; i < arr.length; i++) {console.log(arr[i])}

The the upper bound of the loop is specified as lenght, a typo for the local variable length. At runtime, lenght will evaluate to undefined, so the check i < lenght will always fail, and the loop body is never executed.

Javascript arrays are treated as objects (though they are instances of Array). Hence, when you write arr.lenght, it treats lenght as a property of an object that is undefined. Hence, you don't get an error. It simply tries to get a property that is undefined. Also, in your case, the loop just does not execute as the condition of the loop is never satisfied.

You could easily add new properties to arr object, JavaScript won't warn you about it, instead it will try to find the property you're calling, and if it didn't find anything such result will be undefined, so the comparison is actually i < undefined everytime because you're calling a property that haven't been created on the object. I'll suggest you to read What does "use strict" do in JavaScript, and what is the reasoning behind it?.