if and else

What is a conditional statement?

A conditional statement runs different code depending on whether a condition is true. Two common forms are if and switch.

Basic syntax

The original interactive example checks whether a == b. If so, it runs document.write("a = b"). Otherwise, it checks a + 1 == b and writes "a + 1 = b" if that condition is true. If neither condition is true, it writes "?".

Open the original if/else example on CodePen.

Some values are treated as false in a condition. The original console examples show false, undefined, NaN, null, 0, and an empty string, "". NaN stands for Not a Number and represents an invalid numeric result.

Comparison operators

The operators used in conditions include ===, !==, ==, !=, <, >, <=, and >=.

• === checks strict equality without converting different types.
• !== is its opposite.
• == uses type conversion when appropriate, so 1 and '1' can compare equal.
• != is the opposite of loose equality.
• < and > compare whether one value is less than or greater than another.
• <= and >= include equality in those comparisons.

Loose and strict equality

As noted above, == treats the number 1 and the string '1' as equal after conversion. Strict equality, ===, distinguishes them because their types differ. The second interactive example illustrates the difference:

Open the original equality comparison on CodePen.

Translation note: the source called this final section “the ternary operator,” but its explanation and example concern equality. The heading has been corrected to match the actual content. The screenshot's falsy-value list is introductory rather than exhaustive.

Next, we will look at switch statements.