Javascript Boolean Operator Logical OR

Introduction

The logical OR operator is represented by the double pipe (||) in Javascript: 

let result = true || false;

Logical OR behaves as in the following truth table:

OPERAND 1 OPERAND 2RESULT
true true true
true falsetrue
false true true
false falsefalse

If either operand is not a Boolean, logical OR will not always return a Boolean value.

It does one of the following:

  • If the first operand is an object, then the first operand is returned.
  • If the first operand evaluates to false, then the second operand is returned.
  • If both operands are objects, then the first operand is returned.
  • If both operands are null, then null is returned.
  • If both operands are NaN, then NaN is returned.
  • If both operands are undefined, then undefined is returned.

Like the logical AND operator, the logical OR operator is short-circuited.

If the first operand evaluates to true, the second operand is not evaluated.

Consider this example: 

let found = true; 
let result = (found || someUndeclaredVariable);  // no error 
console.log(result);  // works

The variable someUndefinedVariable is undefined.

Because the variable found is set to true, the variable someUndefinedVariable is never evaluated and thus the output is "true".

If the value of found is changed to false, an error occurs, as in the following example: 

let found = false; 
let result = (found || someUndeclaredVariable);  // error occurs here 
console.log(result);  // this line never executes

We can use this behavior to avoid assigning a null or undefined value to a variable.

Consider the following: 

let myObject = preferredObject || backupObject;

In this example, the variable myObject will be assigned one of two values.

The preferredObject variable contains the value that is preferred if it's available.

The backupObject variable contains the backup value if the preferred one isn't available.

If preferredObject isn't null, then it's assigned to myObject.

If it is null, then backupObject is assigned to myObject.

This pattern is used very frequently in Javascript for variable assignment.




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