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What is an expression?

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An expression is a phrase of JavaScript that a JavaScript engine can evaluate to produce a value. 

For example,

23+45 //produces a value 68
parseInt(12.45)//produces a value 12

The expression x * y evaluates to the product of the values of the expressions x and y. For simplicity, we sometimes say that an operator “returns” a value rather than “evaluates to” a value.

Primary Expressions

The simplest expressions, known as primary expressions, are those that stand alone— they do not include any simpler expressions. Primary expressions in JavaScript are constant or literal values, certain language keywords, and variable references.

1.23 // A number literal
"hello" // A string literal
/pattern/ // A regular expression literal

Arithmetic Expressions

An arithmetic expression is an expression that results in a numeric value. 

23+34      // results 57
3*9   // results 27
 -(-23)   // results 23

Relational Expressions

Relational expressions always evaluate to a boolean value, and that value is often used to control the flow of program execution in if, while, and for statements. JavaScript supports =, ==, and === operators. Be sure you understand the differences between these assignment, equality, and strict equality operators.

Logical Expressions

Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.

Examples of expressions that can be converted to false are those that evaluate to null, 0, NaN, the empty string (“”), or undefined.

The following code shows examples of the && (logical AND) operator.

var a1 =  true && true;     // t && t returns true
var a2 =  true && false;    // t && f returns false
var a3 = false && true;     // f && t returns false
var a4 = false && (3 == 4); // f && f returns false
var a5 = 'Cat' && 'Dog';    // t && t returns Dog
var a6 = false && 'Cat';    // f && t returns false
var a7 = 'Cat' && false;    // t && f returns false

The following code shows examples of the || (logical OR) operator.

var o1 =  true || true;     // t || t returns true
var o2 = false || true;     // f || t returns true
var o3 =  true || false;    // t || f returns true
var o4 = false || (3 == 4); // f || f returns false
var o5 = 'Cat' || 'Dog';    // t || t returns Cat
var o6 = false || 'Cat';    // f || t returns Cat
var o7 = 'Cat' || false;    // t || f returns Cat

The following code shows examples of the ! (logical NOT) operator.

var n1 = !true;  // !t returns false
var n2 = !false; // !f returns true
var n3 = !'Cat'; // !t returns false

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible “short-circuit” evaluation using the following rules:

  • false && anything is short-circuit evaluated to false.
  • true || anything is short-circuit evaluated to true.

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

Note that for the second case, in modern code you can use the new Nullish coalescing operator (??) that works like ||, but it only returns the second expression, when the first one is “nullish”, i.e. null or undefined. It is thus the better alternative to provide defaults, when values like ” or 0 are valid values for the first expression, too.

NOTE: Some expressions, however, have side effects, and their evaluation may affect the result of future evaluations. The assignment operators are the most obvious example: if you assign a value to a variable or property, that changes the value of any expression that uses that variable or property.