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I spent hours just to find out that I misspelt the word .length as .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 doesn't exist, it returns undefined, and 0 < undefined is false.

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])}

EDIT

I said 'javascript is not a strongly typed language' and it is true. But this way of adding new properties it is a feature of prototype-based programming, as @Voo said.

I also said .length=7 it's a bad idea. After reading a little more, in this case I still think it's a little weird to increase the length property after adding elements. Maybe it's fine to truncate, delete elements or empty an array, although in the latter case I would prefer arr= instead of arr.length=0.

There are some interesting examples about length property in the Mozilla documentation.

A JavaScript array's length property and numerical properties are
connected. Several of the built-in array methods (e.g., join(),
slice(), indexOf(), etc.) take into account the value of an array's
length property when they're called. Other methods (e.g., push(),
splice(), etc.) also result in updates to an array's length property.

var fruits = ;

fruits.push('banana', 'apple', 'peach');

console.log(fruits.length); // 3

When setting a property on a JavaScript array when the property is a
valid array index and that index is outside the current bounds of the
array, the engine will update the array's length property accordingly:

fruits[5] = 'mango';

console.log(fruits[5]); // 'mango'

console.log(Object.keys(fruits));  // ['0', '1', '2', '5']

console.log(fruits.length); // 6

Increasing the length.

fruits.length = 10;

console.log(Object.keys(fruits)); // ['0', '1', '2', '5']

console.log(fruits.length); // 10

Decreasing the length property does, however, delete elements.

fruits.length = 2;

console.log(Object.keys(fruits)); // ['0', '1']

console.log(fruits.length); // 2

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.

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.

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?.

Why

Standard JavaScript arrays aren't really arrays at all¹, they're objects, and if you read an object property that doesn't exist (like lenght), you get the value undefined (even in strict mode):

console.log(({}).foo); // undefined

When you use undefined in a relational operation like < or > with a number, it gets converted to a number, but the number value it gets is the special number NaN, which has the bizarre property of always causing comparisons to be false:

console.log(NaN < 0);     // false

console.log(NaN > 0);     // false

console.log(NaN === 0);   // false

console.log(NaN === NaN); // false!!

What you can do about it

Linter tools will often pick these things up in simple cases.

Alternately, TypeScript provides a full static typing layer on top of JavaScript which can catch these sorts of errors.

If you wanted (and this would probably be overkill), you could wrap a Proxy around your objects that threw a proactive error when you tried to read a property that didn't exist:

function proactive(obj) {

  return new Proxy(obj, {

    get(target, propName, receiver) {

      if (!Reflect.has(target, propName)) {

        throw new TypeError(`Property '${propName}' not found on object`);

      }

      return Reflect.get(target, propName, receiver);

    }

  });

}



const a = proactive(["a", "b"]);

a.push("c");

for (let i = 0; i < a.length; ++i) {

  console.log(a[i]);

}

console.log(`Length is: ${a.lenght}`); // Note the typo

.as-console-wrapper {

  max-height: 100% !important;

}

There's a significant runtime penalty, though.

¹ (that's a post on my anemic little blog)