Javascript Boolean Operator Logical AND

Introduction

The logical AND operator is represented by the double ampersand (&&) and is applied to two values: 

let result = true && false;

Logical AND behaves as described in the following truth table:

OPERAND 1 OPERAND 2RESULT
truetrue true
truefalsefalse
false true false
false falsefalse

Logical AND can be used with any type of operand, not just Boolean values.

When either operand is not a primitive Boolean, logical AND does not always return a Boolean value; instead, it does one of the following:

  • If the first operand is an object, then the second operand is always returned.
  • If the second operand is an object, then the object is returned only if the first operand evaluates to true.
  • If both operands are objects, then the second operand is returned.
  • If either operand is null, then null is returned.
  • If either operand is NaN, then NaN is returned.
  • If either operand is undefined, then undefined is returned.

The logical AND operator is a short-circuited operation:if the first operand determines the result, the second operand is never evaluated.

In the case of logical AND, if the first operand is false, no matter what the value of the second operand, the result can't be equal to true.

Consider the following example: 

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

This code causes an error when the logical AND is evaluated, because the variable someUndeclaredVariable isn't declared.

The value of the variable found is true, so the logical AND operator continued to evaluate the variable someUndeclaredVariable.

When it did, an error occurred because someUndeclaredVariable is not declared and therefore cannot be used in a logical AND operation.

If found is set to false, as in the following example, the error won't occur: 

let found = false; 
let result = (found && someUndeclaredVariable);  // no error 
console.log(result);  // works

In this code, the console.log is displayed successfully.

Even though the variable someUndeclaredVariable is undefined, it is never evaluated because the first operand is false.

The result of the operation must be false, so there is no reason to evaluate what's to the right of the &&.




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