JS Mistakes

This chapter highlights some common mistakes made in JavaScript.

Accidentally Using the Assignment Operator

JavaScript programs can produce unexpected results if a programmer mistakenly uses the assignment operator (=) instead of the comparison operator (==) in an if statement.

In the following example, the if statement returns false (as expected) because x is not equal to 10:

let x = 0;
if (x == 10)

This if statement returns true (possibly not as expected) because 10 is considered a truthy value.

let x = 0;
if (x = 10)

This if statement returns false (possibly not as expected) because 0 is considered falsy.

let x = 0;
if (x = 0)

An assignment expression always returns the value that has been assigned.

Expecting Loose Comparison

In a regular comparison, the data type is ignored. This if statement returns true:

let x = 10;
let y = “10”;
if (x == y)

In strict comparison, the data type is taken into account. This if statement returns false:

let x = 10;
let y = “10”;
if (x == y)

In strict comparison, the data type is important. This if statement returns false:

let x = 10;
let y = “10”;
if (x === y)

It’s a common mistake to overlook that switch statements use strict comparison.

This switch case will trigger an alert:

let x = 10;
switch(x) {
case 10: alert(“Hello”);
}

This switch case will not trigger an alert:

let x = 10;
switch(x) {
case “10”: alert(“Hello”);
}