JavaScript and TypeScript Type Conversions 2024

Type Conversions in JavaScript and TypeScript

In JavaScript and TypeScript, values can be converted from one type to another automatically or explicitly.

String Conversion

String conversion occurs when you need to represent a value as a string.

Example in JavaScript:

let num = 42;
console.log(typeof num); // number

num = String(num); // now num is a string "42"
console.log(typeof num); // string

Example in TypeScript:

let num: any = 42;
console.log(typeof num); // number

num = String(num); // now num is a string "42"
console.log(typeof num); // string

• In both examples, the number 42 is converted to the string "42".

Numeric Conversion

Numeric conversion happens automatically in mathematical operations, but you can also explicitly convert values to numbers.

Example in JavaScript:

let bool = false;
console.log(typeof bool); // boolean

let num = Number(bool); // becomes a number 0
console.log(typeof num); // number

Example in TypeScript:

let bool: boolean = false;
console.log(typeof bool); // boolean

let num: number = Number(bool); // becomes a number 0
console.log(typeof num); // number

• In these examples, the boolean false is converted to the number 0.
• If the string cannot be converted to a valid number, the result is NaN.

Example in JavaScript:

let value = Number("not a number");
console.log(value); // NaN

Example in TypeScript:

let value: number = Number("not a number");
console.log(value); // NaN

Numeric Conversion Rules:

Examples:

console.log(Number("   456   ")); // 456
console.log(Number("456abc"));   // NaN
console.log(Number(false));      // 0
console.log(Number(true));       // 1

Boolean Conversion

• Boolean conversion is straightforward. Values that are "empty" become false, and all other values become true.

Example in JavaScript:

console.log(Boolean(123)); // true
console.log(Boolean(0)); // false

console.log(Boolean("JavaScript")); // true
console.log(Boolean("")); // false

Example in TypeScript:

console.log(Boolean(123)); // true
console.log(Boolean(0)); // false

console.log(Boolean("JavaScript")); // true
console.log(Boolean("")); // false

• Even the string "0" and strings with spaces are true.

Examples:

console.log(Boolean("0")); // true
console.log(Boolean(" ")); // true

Summary

The three primary type conversions in JavaScript and TypeScript are:

String Conversion: Can be done with String(value).

• String Conversion: Can be done with String(value).
• Examples: String(42) // "42", String(null) // "null"

Numeric Conversion: Can be done with Number(value).

Rules:

• undefined becomes NaN
• null becomes 0
• true becomes 1, false becomes 0
• Strings are converted to numbers if possible, otherwise NaN

Boolean Conversion: Can be done with Boolean(value).

Rules:

• 0, null, undefined, NaN, "" become false
• All other values become true

Most of these rules are intuitive, but some can be tricky:

• undefined converts to NaN in numeric conversion.
• "0" and strings with spaces convert to true in boolean conversion.

By understanding these conversions, you can handle data types more effectively in your JavaScript and TypeScript programs.

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