Reference · Maps

merge

merge(map | null, map | null) -> map

What it does

One map made of two: every entry of the first, then every entry of the second, which wins where both have a key. The result is a new map; neither argument changes. It is { ...a, ...b } as a function, for a pipeline or a reduce.

Examples

let defaults = { page: 1, size: 20, sort: "name" }let query = { page: 3, sort: null }print(merge(defaults, query))print(defaults)

{ page: 3, size: 20, sort: null }{ page: 1, size: 20, sort: "name" }

Edge cases

• The right side wins, null values included: a null on the right replaces a value on the left. Filter it out first when null means "not given".
• Order: keys of the first map in their order, then the new keys of the second; a key in both keeps the first map's position.
• null on either side is an empty map; merge(null, null) is {}.
• Only two maps: a third argument is a runtime error (below). Fold more with reduce.
• The order is the same in the browser, keys that look like numbers included: merge({ b: 1 }, { "2": 2 }) is { b: 1, "2": 2 } on both sides.

print(merge({ a: 1, b: 2 }, { b: null }))print(merge({ a: 1, b: 2 }, { c: 3, a: 0 }) | keys)print(merge({ a: 1 }, null), merge(null, { a: 1 }), merge(null, null))print([{ a: 1 }, { b: 2 }, { c: 3 }] | reduce({}, merge))print(merge({ b: 1 }, { "2": 2 }) | keys)

{ a: 1, b: null }["a", "b", "c"]{ a: 1 } { a: 1 } {}{ a: 1, b: 2, c: 3 }["b", "2"]

Errors

An argument that is a list, a string or a number is a runtime error, not an error value. Lists are joined with [...a, ...b].

print(merge({ a: 1 }, [1]))

line 1: `merge` expects a map as argument 2, got list

A third map is a runtime error, and one for quillo check when the call is written out:

print(merge({ a: 1 }, { b: 2 }, { c: 3 }))

line 1: `merge` takes 2 arguments, got 3: merge(a, b)

Chaining

let base = { color: "red", size: "m" }print(base | merge({ size: "l" }))print({ ...base, size: "l" } == merge(base, { size: "l" }))

{ color: "red", size: "l" }true

See also

set, remove, get, keys, reduce

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