Implicit type coercion

Learn how implicit type coercion works with +, == and in if conditions.

Implicit type coercion

Sometimes JavaScript changes a value's type on its own while performing an operation. This is called implicit type coercion.

Example: in "5" + 3 the number 3 is turned into a string and the result is "53". That is because the + operator concatenates when one side is a string. But in "5" - 3 both sides are turned into numbers and the result is the number 2.

Conversion by operator

Knowing how each operator behaves prevents confusion. The + operator concatenates when one side is a string: "a" + 1 → "a1".

The -, * and / operators always expect numbers; they convert strings to numbers: "10" - 2 → 8, "6" * "2" → 12. When a string cannot become a number the result is NaN: "abc" * 2 → NaN.

Exercise Three-operator demo

Define a function called showCoercion. It should take a and b parameters. In the body write three lines: console.log(a + b), console.log(a - b) and console.log(a * b).

Show example solution

function showCoercion(a, b) {
  console.log(a + b);
  console.log(a - b);
  console.log(a * b);
}

Exercise Call showCoercion with two type orders

The showCoercion function below is defined. Now call it twice: showCoercion("5", 3) and showCoercion(5, "3"). Put each call on its own line.

Show example solution

function showCoercion(a, b) {
  console.log(a + b);
  console.log(a - b);
  console.log(a * b);
}

showCoercion("5", 3);
showCoercion(5, "3");

Test yourself

What does the expression "5" + 3 return?
```
console.log("5" + 3);
```
- 8 (number)
- 53 (number)
- "53" (string)
- NaN
Show answer

"53" (string) — Correct. The + operator turns the other side into a string and concatenates when one side is a string; the result is "53".

Complete the concatenation expression

let result = "5" + ____;
console.log(result);

Show answer

let result = "5" + 3;
console.log(result);

Match the expression to its result

"5" + 3

"53"
"5" - 3

2
"5" * 2

10
"abc" * 2

NaN

Next lesson: Difference between == and ===

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