JS: ternary expressions

Published on: December 26, 2014

Tags: js and interview-questions

Why is it called a Ternary expression, what does the word “Ternary” indicate?

Let’s answer the second question first: what does the word “ternary” indicate? According to Wikipedia the word “ternary” comes from the n-ary word setup. Other examples of n-ary words are unary and binary. All of these (including ternary) are operands. The prefix section of their name lists how many inputs the operand accepts.

A unary operand accepts one parameter, e.g. -1, where - is the operand, and 1 is the parameter.

A binary operand accepts two parameters, e.g. 2 + 3, where + is the operand, and 2 and 3 are the parameters.

So a ternary operand accepts three parameters.

In programming the ternary operand we use is a rewrite of an if statement. Before we write an actual ternary, we'll just take a quick look at an if statement:

if(conditional) { // one
    truethy_block // two
} else {
    falsey_block // three

You can see there are three sections to an if statement. Let’s write them as a property ternary expression:

conditional ? truethy_block : falsey_block

All the same code is there, but it’s arranged slightly differently. The ternary’s operand looks like ?:.

In JS ternarys are often used for assignment:

is_sunny = true;
var weather = is_sunny ? ’sunny' : 'Cloudy';
console.log(weather); // logs ’sunny'

They can also be used for very short conditional statements. But be wary of using them for long or complex logic as they are harder to read than traditional statements.


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