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TypeScript error

TS2556: Spreading an array into fixed parameters

  • Error TS2556
  • Tested with TypeScript 5.9

error TS2556: A spread argument must either have a tuple type or be passed to a rest parameter. is reported for a call like centerOn(...office) when office is an ordinary array. The checker knows it is a number[], an array of any length, so it cannot tell whether it supplies the two arguments centerOn needs.

In JavaScript an array literal is an array of its element type, not a fixed-length tuple. Saying "exactly these two numbers" takes a tuple type in JSDoc, or a const cast.

At a glance

CodeTS2556
MessageA spread argument must either have a tuple type or be passed to a rest parameter.
TypeScript's categoryerror
Listed underTypes do not match
Turned on by No option: every checked file reports it
In .js files Reported when the file is checked: checkJs in jsconfig.json, or // @ts-check at the top of the file

The message is TypeScript's own text from its diagnostic messages file (Apache-2.0).

What it looks like

map.js Error confirmed 5.9.3 TypeScript 5.9.3, run 21 October 2026: map.js(11,10): error TS2556: A spread argument must either have a tuple type or be passed to a rest parameter. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. map.js(11,10): error TS2556: A spread argument must either have a tuple type or be passed to a rest parameter. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. map.js(11,10): error TS2556: A spread argument must either have a tuple type or be passed to a rest parameter. checked 21 October 2026 
/**
 * @param {number} lat
 * @param {number} lng
 */
export function centerOn(lat, lng) {
  return `${lat.toFixed(4)},${lng.toFixed(4)}`;
}

const office = [40.7128, -74.006];
centerOn(...office);error TS2556: A spread argument must either have a tuple type or be passed to a rest parameter.
Spreading an array into fixed parameters.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}

What causes it

  • The spread value's type is an array type (number[]) and the function has a fixed list of parameters.

How to fix it

Give the array a tuple type, or cast it to const

/** @type {[number, number]} */ before the declaration makes it a pair; /** @type {const} */ ([41.8781, -87.6298]) makes it a read-only tuple of those exact values. Either one lets the spread fill the parameters.

map.js TypeScript 5.9.3 TypeScript 5.9.3, run 21 October 2026: No errors. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. No errors. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. No errors. checked 21 October 2026 
/**
 * @param {number} lat
 * @param {number} lng
 */
export function centerOn(lat, lng) {
  return `${lat.toFixed(4)},${lng.toFixed(4)}`;
}

/** @type {[number, number]} */
const office = [40.7128, -74.006];
centerOn(...office);

const warehouse = /** @type {const} */ ([41.8781, -87.6298]);
centerOn(...warehouse);

/** @param {...number} values */
export function average(...values) {
  return values.reduce((sum, v) => sum + v, 0) / values.length;
}

const scores = [3, 4, 5];
average(...scores);
A tuple type, or a rest parameter.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}

Or give the function a rest parameter

If the function works with any number of values, declare it that way: @param {...number} values with ...values in the code. Arrays then spread into it freely.

Gotchas

  • Built-ins with rest parameters, such as Math.max(...values) and console.log(...parts), accept any spread array; Math.atan2(...pair) does not.
  • A const cast makes the tuple readonly; passing it to a parameter typed as a mutable array is then an error. That only matters when the function expects an array, not separate arguments.

  • Type expression

    Tuples

    A tuple is an array with a fixed length and a type per position. Writing tuples in JSDoc, with named, optional and rest elements, and when you need one.

  • Type expression

    const assertions

    /** @type {const} */ (value) is the JSDoc form of as const, since TypeScript 4.5: literal types kept, properties read-only, arrays turned into read-only tuples.

  • Type expression

    Rest parameters

    A rest parameter in JSDoc is written @param {...string} tags: the element type after three dots. Inside the function the parameter is a string[]. Examples.

  • TypeScript error

    TS2554: The wrong number of arguments

    TS2554 and TS2555 mean a call passes too few or too many arguments. In JavaScript, @param tags make parameters required; brackets make one optional.

Sources

Last reviewed 20 October 2026.