Overall Goal:

• Defaults aur config ko merge karna.
• Config values defaults ko override karengi.
• New object return (mutation avoid).

Single Line Solution:

• const withDefaults = (config, defaults) => ({ ...defaults, ...config });

Part 1: Spread defaults

• { ...defaults } → defaults object ko spread karo:
• ... → spread operator: object ke properties ko copy karta hai.
• Example: {...{a: 1, b: 2}} → {a: 1, b: 2}.

Part 2: Override with config

• ...config → config object ko spread karo:
• Agar same key hai → config value override karegi.
• Agar new key hai → add ho jayegi.

Spread order matters:

• {...defaults, ...config} → defaults pehle, config baad:
• Same keys → config wins (override).
• Different keys → dono merge ho jayengi.

Example:

• withDefaults({b: 20}, {a: 1, b: 2}) → {a: 1, b: 20}.
• b override ho gaya, a default se aaya.

Real world:

• Feature flags: defaults + user config merge.
• Settings: default values + user preferences.
• Component props: default props + passed props.