@suchipi/has-shape

Very tiny (~200B before minification/compression) function that checks if an object/array/value is shaped like another, with TypeScript type refining.

import { hasShape } from "@suchipi/has-shape";

const someObj = {
  sheep: true,
  cows: ["betsy", "donna"],
  tags: {
    something: {
      active: true,
      description: "Something",
    },
    anotherThing: {
      active: false,
      description: "Another thing",
    },
  },
};

hasShape(someObj, { tags: { something: { active: true } } }); // true
hasShape(someObj, { tags: { something: { active: false } } }); // false
hasShape(someObj, { tags: { something: { blahbla: true } } }); // false; no blahbla property present
hasShape(someObj, { cows: { 0: "betsy" } }); // true; property keys are used for both objects and arrays
hasShape(someObj, { cows: { 0: "donna" } }); // false; donna is at property '1'
hasShape(someObj, { cows: { length: 2 } }); // true; reading property "length" from Array returns matching value
hasShape(someObj, { cows: { length: 4 } }); // false
hasShape(someObj, { tags: {} }); // true; validates it's a non-null object, but nothing else

Logic

The function behaves as follows:

• Consider a function hasShape which receives input and shape, defined as follows:
• If input is a primitive value, return input === shape.
• Otherwise, return true if hasShape(input[property], shape[property]) is true for every own property in shape.

TypeScript

hasShape will tell TypeScript that the object has the specified shape if you put it in an if, like so:

import { hasShape } from "@suchipi/has-shape";

function runJob(
  options?:
    | { kind: "User"; name: string; id: number }
    | { kind: "Ticket"; ticket: string }
) {
  // `options` could be one of a few things
  if (hasShape(options, { kind: "User" })) {
    // Within this block, TypeScript knows `options` is `{ kind: "User", name: string, id: number }`,
    // because that's the only value in the union that could have returned true from hasShape.
  } else {
    // Within this block, TypeScript knows `options` could be `undefined` or `{ kind: "Ticket", ticket: string }`
    // here, and knows it could not be `{ kind: "User", name: string, id: number }`, because if it was,
    // the other block would have been taken instead of this one.
  }
}

This is most useful for deep structures with many different potential but distinguishable shapes, like ASTs:

import { traverse } from "@babel/traverse";
import { parse } from "@babel/parser";
import { hasShape } from "@suchipi/has-shape";

const someCode = "...";
const ast = parse(someCode);
traverse(ast, {
  CallExpression(path) {
    if (
      hasShape(path.node, {
        callee: {
          type: "Identifier",
          name: "require",
        },
        arguments: {
          length: 1,
          0: {
            type: "StringLiteral",
            value: "hello",
          },
        },
      })
    ) {
      // We now know it's a path pointing to a node with the shape `require("hello")`,
      // and TypeScript will let us treat it as such:
      console.log(path.node.callee.arguments[0].value); // No type or runtime errors!
    }
  },
});

If your shape is gonna be stored in a variable instead of being put directly in the function call, put as const at the end of the variable declaration, otherwise TypeScript won't "know" everything (ie. type refinement might not work well enough):

const targetShape = { blah: { wow: { isCool: true } } } as const;

hasShape(something, targetShape);

Additionally, if you're using an older version of TypeScript, you might need to add as const even if you're not storing it in a variable:

// If TypeScript doesn't seem to behave correctly when you do this:
hasShape(something, { blah: { wow: { isCool: true } } });
// Then do this instead:
hasShape(something, { blah: { wow: { isCool: true } } } as const);

License

MIT