ts-deep-pick

ts-deep-pick is a TypeScript type-only utility package that allows for type-safe, deep picking/omitting of properties from nested object types. Powered by the template literal type released in TypeScript 4.1, the type DeepPick will offer all combinations of nested pick/omit string patterns that will then return the deeply picked/omit type according to the deep-picking grammar.

Usage

DeepPick

import { DeepPick } from "ts-deep-pick";

interface FooBar {
  foo: ABC;
  bar: ABC;
}

interface ABC {
  a: number;
  b: number;
  c: number;
}

// Result type: { foo: { a: number } };
// including a property "positively" omits the exluded properties.
type FooWithJustA = DeepPick<FooBar, "foo.a">; // "." joins nested properties

// Result type: { foo: { a: number, b: number } };
type FooWithJustAB = DeepPick<FooBar, "foo.a" | "foo.b">;properties

// Result type: { foo: { a: number, b: number } };
type FooWithoutC = DeepPick<FooBar, "foo.!c">; // "!" omits a property

// Result type: { bar: ABC };
type Bar = DeepPick<FooBar, "!bar">; // "!" omits a property

// Result type:
// {
//    foo: { a: number, b: number },
//    bar: ABC
// };
// "~" mutates the property's nested membership without explicitly picking
// the property and thereby omitting the remaining.
type FooBarWithoutFooC = DeepPick<FooBar, "~foo.!c">;

// Result type: Array<{ foo: ABC }>
type ArrayWithFoo = DeepPick<FooBar[], "[].foo"> // "[]" accesses an array item.

// Result type: FooBar
type ArrayWithFoo = DeepPick<FooBar, "*"> // "*" is the "identity" operand

DeepPickPath

Returns the possible path that can be used to passed into DeepPick for a given structure.

import { DeepPickPath } from "ts-deep-pick";

// assume FooBar as defined earlier.
// "foo" | "bar" | "!foo" | "!bar" | "~foo.a" | "~foo.!a" | ...
type Path = DeepPickPath<FooBar>;

Behavior

• Union paths are treated like a combination of paths. For example, "a"|"b" would mean "pick properties a and b".
• At each level, the "picked set" is determined before omitting and mutating. If the paths at the given level include at least one explicitly picked key ("a") then these keys and the mutations ("~a") become the picked set. In this case, omits are redundant because they are either no-op (omitting keys that aren't picked) or they make the picks no-op (omitting keys that are picked).
• If the paths at the given level consist only of omits ("!a") and mutations ("~a"), then the omit operation takes place on the full set of keys.

Examples

• "a" | "b" will pick just properties a and b.
• "a" | "!a" will pick nothing (a is picked but is then omitted.)
• "~a.foo" will pick everything at the outermost level, and pick foo as the only property of a.
• "!a" | "~b.foo" will pick everything except for a at the outermost level, and modifies b such that only foo is available there.

Grammar

The character tokens ., !, ~, [] and * define the property separator, omit prefix, mutation prefix, array index property and the pass-through (return the original type) operand respectively. These tokens can be replaced by defining the following interface and passing it as the extra type parameter to DeepPick and DeepPickPath.

interface DeepPickGrammar {
  array: string;  // default: "[]"
  prop: string;   // default: "."
  omit: string;   // default: "!"
  mutate: string; // default: "~"
  glob: string;   // default: "*"
}

Example:

import { DefaultGrammar } from "ts-deep-pick";

interface G extends DeepPickGrammar {
  props: ":"; // Now use ":" as property separator
}

// Result type: { foo: { a: number } };
type FooWithJustA = DeepPick<FooBar, "foo:a", G>;

AmbiguousProp<G = DefaultGrammar>

Returns the template literal type that represents any string that -- if used as a property name -- would be ambigous given the set of character tokens.

// Result type: "a"
// ("!a" is ambiguous because of the "!" character).
// ("a.b" is ambiguous because of the "." character)
type ValidProps = Exclude<"a" | "!a" | "a.b", AmbiguousProps>

// Result type: "a" | "a.b"
// ("a:b" would have been ambiguous now because of custom grammar G defined
// earlier).
type ValidProps = Exclude<"a" | "!a" | "a.b", AmbiguousProps<G>>