why Object.prototype.toString return [object Object] [closed]

Answer 1

See answers from @Leo and @Joel Gregory for an explanation from the spec. You can display an objects' contents using JSON.stringify, e.g.:

const log = (...args) => document.querySelector("pre").textContent += `${args.join("\n")}\n`;
const obj = { name: 'John' }

log(obj.toString());
log(JSON.stringify(obj));