javascript_engineer_works.md

JavaScript engine fundamentals: Shapes and Inline Caches

link: https://mathiasbynens.be/notes/shapes-ics

JavaScript Engines:

• V8; NodeJS
• SpiderMoney; SpiderNode
• Charkra; Node Charkra
• JSC: powering Safari & React Native

JS source code -> parser -> Abstract Sytax Tree

Interpreter -> bytecode | (when func gets hot) | (deoptimizing) optimizing compiler -> optimized code

All js object are dictionaries with string keys, and the string keys map to a value called Property attributes. [MDN-Object.defineProperty] (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty)

Property attributes can store:

• Writeable: the properity can be reassigned to.
• Enumberable: the properity can show up in fo-in loop.
• Configurable: it is deletable properity.

obj = Object.defineProperties({}, {
  a: {
    value: 1,
    writable: false,
    enumerable: true,
    configurable: false
  },
});

obj.a = 11;
console.log(obj.a); // 1

delete obj.a;
console.log(obj.a); // 1

for(let x in obj) {
  console.log(x); // a
}

• The upper limit size of the array is 2^32 - 1

new Array(2**32)
// Uncaught RangeError: Invalid array length

Sharp

• Always initialize objects in the same way, so that they don't end up with different sharps

Inline Cache (IC)

The ingredient to make JS run fast.

• Don't mess up with the property attributes of array elements, so that they can be stored and operated efficiently.
• Avoid using Object.defineProperty in array index
• It blows up the element backing store and turns the whole thing into an object