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Use Number(text) when the entire string should represent a number; use parseInt(text, 10) or parseFloat(text) only when you deliberately want to read a numeric prefix. Conversion happens at runtime, so a TypeScript assertion such as text as number does not convert the string. For untrusted input, check the result for NaN.
Choose the conversion method that matches the input
JavaScript, which TypeScript compiles to, provides several ways to turn text into a numeric value. They differ in whether they require the whole value to be numeric or accept a valid numeric prefix.
| What you need | Method | Behavior |
|---|---|---|
| Convert the whole expected numeric string | Number(text) |
Use when trailing text should not be accepted as part of the number. Check the result for NaN. |
| The same coercion in a compact expression | +text |
Unary plus performs numeric coercion. Number(text) is often clearer to readers new to the language. |
| Read an integer prefix | parseInt(text, 10) |
Reads an integer from the start, dropping a decimal fraction and potentially ignoring trailing non-integer text. Specify radix 10 for ordinary decimal input. |
| Read a decimal prefix | parseFloat(text) |
Reads a decimal or exponent prefix and may ignore trailing nonnumeric text. |
| Preserve a very large integer | BigInt(text) |
Use for integer strings when JavaScript Number precision is insufficient; do not convert through Number first. |
Convert the whole string with Number
For a value expected to contain a number rather than a number followed by a unit or other suffix, use Number():
const text: string = "42";
const value: number = Number(text);
Unary plus is a shorter alternative with the same general coercion intent:
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const value = +"42";
Choose Number() when clarity matters, especially in code that other developers will maintain. Neither form is a substitute for validating user input: conversion can produce NaN.
Use parseInt or parseFloat for a numeric prefix
The parse functions are useful when the goal is specifically to extract a number from the beginning of text. They do not require the entire string to be numeric, so they are not strict validation for a complete field.
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Parse an integer prefix
const count = parseInt("42px", 10); // 42
const truncated = parseInt("1.9", 10); // 1
The radix argument tells parseInt which numeral base to use. For ordinary decimal input, pass 10. MDN also documents that parseInt("123_456", 10) returns 123, illustrating that characters after the valid integer prefix are not necessarily rejected. MDN’s parseInt() reference describes this prefix behavior and precision limitations.
Parse a decimal prefix
const measurement = parseFloat("3.14kg"); // 3.14
This can be useful for extraction, but it does not establish that the whole input is a valid measurement. For example, parseFloat("3.14some text") returns 3.14. See MDN’s parseFloat() reference for its parsing behavior.
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Check for failed conversions
Both parseInt() and parseFloat() return NaN when they cannot find an acceptable numeric prefix. Whole-value conversion with Number() can also yield NaN. Use Number.isNaN() to detect that result:
function toNumber(input: string): number {
const value = Number(input);
if (Number.isNaN(value)) {
throw new Error("Expected a numeric value");
}
return value;
}
Do not use a truthiness check such as if (!value) to decide whether conversion succeeded: zero is a valid numeric result. If a field must follow a particular format, first define its accepted input grammar and validate the complete value rather than relying on a prefix parser.
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TypeScript types do not convert values
TypeScript adds static type checking to JavaScript; it does not change the runtime value simply because a type says otherwise. The primitive types string and number describe different values. An assertion such as text as number, or even text as unknown as number, changes the compiler’s view but leaves the runtime string untouched. Use a runtime conversion function instead. The TypeScript Handbook’s Everyday Types and TypeScript for JavaScript Programmers explain the relationship between TypeScript’s type system and JavaScript values.
TypeScript 5.0 release notes show unary plus as an explicit way to coerce an operand before comparison and describe compiler diagnostics for potentially implicit string-to-number coercion in relational comparisons. Those diagnostics do not make assertions into runtime conversions. See the TypeScript 5.0 release notes.
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Use BigInt when integer precision matters
JavaScript has one ordinary Number type rather than separate integer and floating-point types at the language level. As a result, converting integer text beyond the precise range of Number can lose integer precision. Do not use parseInt() as a precision-preserving large-integer parser: MDN demonstrates that parsing "900719925474099267n" produces a rounded value.
const exactInteger = BigInt("900719925474099267");
Pass the integer string directly to BigInt() rather than converting it through Number. MDN’s parseInt() reference discusses the large-integer example; the TypeScript Handbook covers JavaScript’s numeric types.
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