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Logical Operators

JavaScript Fundamentals: Logical Operators

What makes the logical || (OR), && (AND), ! (NOT) operators unique?​

View Answer:
Interview Response: You can apply logical operators to values of any type; logical operators are not limited to a Boolean type.

If an operand is not a Boolean, what does the logical || OR do?​

View Answer:
Interview Response: If an operand is not a Boolean, it converts to a Boolean for the evaluation. For instance, the number 1 gets treated as true, and the number 0 is false.

Code Example:

/* Logical OR returns the first truthy value,
 *  or the last value if there are none
 */

console.log('1' || '0'); // returns 1

console.log(1 || 0); // returns 1

console.log(0 || 0); // returns 0

How does logical OR “||” function?​

View Answer:
Interview Response: The OR || operator does the following:

  1. Operands evaluate from left to right.
  2. Converts each operand to a Boolean value, and if the result is true, the program terminates and returns the operand's original value.
  3. Returns the final operand if all operands get evaluated (i.e., all were false).

Code Example:

alert(1 || 0); // 1 (1 is truthy)

alert(null || 1); // 1 (1 is the first truthy value)
alert(null || 0 || 1); // 1 (the first truthy value)

alert(undefined || null || 0); // 0 (all falsy, returns the last value)
note

Logical OR “||” finds the first truthy value or the last value if there are none.

What is the definition of a short-circuit evaluation in JavaScript?​

View Answer:
Interview Response: When JavaScript evaluates an OR expression, JavaScript short-circuits by not proceeding to the second operand if the first operand is true.

Code Example:

true || alert('not printed');
false || alert('printed');

What does the Logical && (AND) return?​

View Answer:
Interview Response: Logical && (AND) returns true if both operands are truthy and false otherwise.

Code Example:

alert(true && true); // true
alert(false && true); // false
alert(true && false); // false
alert(false && false); // false

What type of conversion does the Logical && (AND) perform?​

View Answer:
Interview Response: Logical && (AND) converts all operands to Boolean values, true or false.

What steps does JavaScript perform when using the Logical && (AND) operator?​

View Answer:
Interview Response: The AND && operator performs the following functions:

  1. Operands evaluate from left to right.
  2. Converts each operand to a Boolean value, and if the result is false, the program terminates and returns the operand's original value.
  3. It returns the final operand if all operands get evaluated (i.e., all were true).

Code Example:

// if the first operand is truthy,
// AND returns the second operand:
alert(1 && 0); // 0
alert(1 && 5); // 5

// if the first operand is falsie,
// AND returns it. The second operand is ignored
alert(null && 5); // null
alert(0 && 'no matter what'); // 0

What are the differences between Logical || (OR) and Logical && (AND) operators?​

View Answer:
Interview Response: The difference is that AND returns the first falsie value while OR returns the first truthy.

Which of the two, Logical || (OR) and && (AND) operators have the higher operator precedence?​

View Answer:
Interview Response: The AND operator has higher precedence than the OR operator.

Technical Response: The precedence of the AND && operator is greater than that of the OR || operator. As a result, the code a && b || c && d is effectively the same as if the && expressions were enclosed in parentheses: (a && b) || (c && d).

Can you replace conditional if with OR (||) or && (AND)?​

View Answer:
Interview Response: Technically, Yes, but it is not recommended because it reduces readability.

Technical Response: Technically, we can do it, which goes against recommendations. Although the variant with && appears shorter, an if statement is more prominent and tends to be more readable.

Code Example:

let x = 1;

x > 0 && alert('Greater than zero!');

// the if example is cleaner and obvious

let x = 1;

if (x > 0) alert('Greater than zero!');

What steps does JavaScript perform when implementing the Logical! (NOT) operator?​

View Answer:
Interview Response: The operator accepts a single argument, converts it to a Boolean, and returns the inverse value.

Technical Response: The operator accepts a single argument and does the following:

  1. Converts the operand to a Boolean type: true/false.
  2. Returns the inverse value.

Code Example:

alert(!true); // false
alert(!0); // true

What happens when we apply the !! (DOUBLE NOT) to a value?​

View Answer:
Interview Response: The value is inverted twice, returning it to its original Boolean value.

Technical Response: The first NOT changes the value to Boolean and returns the inverse, whereas the second NOT reverses the process. Finally, we have a simple Value-to-Boolean translation (Boolean conversion).

Code Example:

alert(!!'non-empty string'); // true
alert(!!null); // false

Is there a built-in object that performs in the same fashion as the !! (DOUBLE NOT) operator?​

View Answer:
Interview Response: The Boolean object produces the same result as the double not.

Technical Response: The built-in Boolean object performs this in the same fashion as the !! (DOUBLE NOT) operator.

Code Example:

alert(!!'non-empty string'); // true
alert(!!null); // false

// Example of the Boolean method
alert(Boolean('non-empty string')); // true
alert(Boolean(null)); // false

What has the highest operator precedence over all the logical operators?​

View Answer:
Interview Response: Logical NOT is the highest of all logical operators. It always executes first.

Technical Response: The precedence of NOT ! is the highest of all logical operators, so it always executes first, before && (AND) or || (OR).