JavaScript Trick Question: Adding and Subtracting Booleans with Numbers – Deloitte Interview Example

Our Javascript Code

console.log(1 + false); // ?
console.log(1 + true);  // ?
console.log(1 - false); // ?

All of these involve number + boolean or number - boolean.

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Step-by-Step: Type Coercion in JavaScript

In JavaScript, when you use arithmetic operators like + and -, non-number values are converted to numbers if possible.

Boolean to Number Conversion:

true   →    1

false  →    0

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💡 Expression 1: 1 + false

Step-by-Step:

1. false is converted to 0
2. 1 + 0 = 1

Output:

1

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💡 Expression 2: 1 + true

Step-by-Step:

1. true is converted to 1
2. 1 + 1 = 2

Output:

2

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💡 Expression 3: 1 - false

Step-by-Step:

1. false is converted to 0
2. 1 - 0 = 1

Output:

1

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🧠 Why Does JavaScript Do This?

Because JavaScript is dynamically typed and loosely typed, it automatically tries to convert values to the most suitable type for the operation.

• When you use + or -, JavaScript assumes you're doing math, so it coerces booleans to numbers.

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Summary Table

Expression	Converted Expression	Result
1 + false	1 + 0	1
1 + true	1 + 1	2
1 - false	1 - 0	1

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✔ Final Answer:

D) 1 2 1

This question appeared in the Deloitte 2nd round of interview, testing the candidate’s knowledge of type coerce of boolean when we use plus (+) and minus (-) operator with number in JavaScript.