📜 JS Promise: Basic to Advanced

Core Concepts

• then and catch are promise handling methods

• .then() activates when resolve() is called → Promise moves to "Fulfilled" state.

• .catch() activates when reject() is called → Promise moves to "Rejected" state.

• .then() and .catch() are available on the prototype chain (__proto__) of a Promise instance.

javascript
instance.then(v => {
    console.log("2 .then fullfilled promise Value is", v);
});

• Every .then() attaches separately to the original promise.

• No "info sharing" between different .then() calls unless you chain them.

• It creates a brand new Promise internally.

• This new Promise waits for instance to settle.

• It does NOT know anything about other .then() calls on the same instance.

• Memory-wise: Each .then() holds its own closure (callback function and context) — so yes, it has its own "memory" to track things separately.

• .then().then() - chaining , and share info in between

• Log Promise immediately ➔ shows .

• Expand Promise ➔ you might see fulfilled already.

• Because logging is sync, but Promise settling is async. Event loop is still processing promise in micro task portion

• Chaining of Promise handler methods - If you chain .then or .catch multiple time it will also slow the execution as well. Bcz they get resolve or rejected one by one.

• Note - resolve() or reject() the promise doesn't means the function get stopped , if you write something after the resolve or reject the promise , that gonna run as well. Only return resolve or reject stop that function right there.

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Mastering JavaScript! 🚀