Today I learned

JS: Null Coalescence Assignment (??=) (2025-11-30)

function memoize(fn) {
    const known = [];
    return (num) => (known[num] ??= fn(num));
}

It's beautiful.

Javascript: variable keys in maps

let x = "apple";

return { [x]: 10 }; // { "apple": 10 }

return { ...state, [x]: 10 };

return { ...state, apples: { [fav]: 10 } };

Javascript: forward declaration (2019-08-20)

Obsolete! Functions are now hoisted automatically.

Many linters check for functions to exist before they're referenced. With circular calling, you need to declare a function before it's defined. TIL this is called forward declaration.

let bind = function bindForward() {};

function ajax(url) {
    load(url).then(bind);
}

bind = function bind() {
    $("a").click(ajax);
};

Javascript: Destructuring (2019-08-08)

When destructuring, JS only assigns the deepest nested keys:

const req = { query: { person: { first: "Chris", last: "Hallberg" } } };

const {
    query: {
        person: { first, last },
    },
} = req;

console.log(query); // undefined
console.log(person); // undefined
console.log(first); // "Chris"
console.log(last); // "Hallberg"

You can also assign names that don't match the keys you're grabbing. Just be careful with your brackets.

const req = { query: { data: "apples" } };

// rename query to data
var { query: data } = req;
console.log(data); // { data: "apples" }

// destruct data from query
var {
    query: { data },
} = req;
console.log(data); // apples

Warning: In browsers, name seems to be assigned to "[object Object]" somewhere along the line, so be careful.

Javascript: Destructuring as parameters

2019-09-19

I learned today that you can destructure variables as part of a function declaration or as part of a for-of loop:

function getHeight({ height }) {
    return height;
}

getHeight({ width: 10, height: 200, depth: 7 }); // 200

const points = [
    { x: 3, y: 4 },
    { x: 5, y: 5 },
];

for (let { x, y } of points) {
    console.log(`distSq = ${(x ** 2, y ** 2)}`);
}

// distSq = 25

// distSq = 50

Javascript: Destructuring as default parameters

2019-09-24

It's been a good month for destructuring. Building on the previous revelation, you can set parameter defaults with destructuring.

function showConfig({ tabSize = 2, theme = "Nord" } = {}) {
    console.log(`Theme: ${theme}, Tab Size: ${tabSize}`);
}

showConfig({ tabSize: 4, theme: "Monokai" }); // Theme: Monokai, Tab Size: 4

showConfig({ theme: "Monokai" }); // Theme: Monokai, Tab Size: 2

showConfig(); // Theme: Nord, Tab Size: 2