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Data Types & Coercion
beginnerJavaScript has a small set of primitives and automatic type conversion rules. Understanding coercion helps you predict equality checks, arithmetic, and truthiness in conditionals.
🎮Interactive Visualization
Type Coercion Lab
Pick values and operators to see how JavaScript converts types.
Left value
Operator
Right value
7=="7"=true
numberstringboolean
What JS does
- Number == String: string is coerced to number
- "7" → Number("7") = 7
- Then 7 == 7 is compared
Key Points
- Seven primitives: string, number, boolean, null, undefined, symbol, bigint
- typeof null is "object" (legacy quirk)
- Loose equality (==) coerces types, strict (===) does not
- + concatenates when either operand is a string
- Other math operators convert operands to numbers
- Truthy/falsy rules drive conditionals and &&/||
💻Code Examples
Loose vs strict equality
'0' == 0 // true
0 == false // true
null == undefined // true
'0' === 0 // false
0 === false // false
null === undefined // falseLoose equality coerces types, strict equality does not
The plus operator
'5' + 1 // "51"
5 + '1' // "51"
true + '1' // "true1"
5 + 1 // 6
true + 1 // 2 (true -> 1)+ concatenates when a string is involved, otherwise adds numbers
Numeric operators
'5' - 1 // 4
'5' * '2' // 10
'five' - 1 // NaNMath operators other than + convert operands to numbers
Truthiness
Boolean(0) // false
Boolean('') // false
Boolean('0') // true
Boolean([]) // trueConditionals use truthiness, not strict booleans
typeof quirks
typeof null // "object"
typeof undefined // "undefined"typeof null is a long-standing JavaScript quirk
Common Mistakes
- Using == and expecting no type conversion
- Assuming empty strings are truthy
- Forgetting that null and undefined only equal each other with ==
- Assuming typeof null returns "null"
Interview Tips
- Explain the difference between == and === with examples
- Know the falsy values: false, 0, "", null, undefined, NaN
- Be ready to reason about + with strings and numbers