A function is known as pure functions if it passes two tests:

𝟬. 𝗦𝗮𝗺𝗲 𝗶𝗻𝗽𝘂𝘁𝘀 𝗮𝗹𝘄𝗮𝘆𝘀 𝗿𝗲𝘁𝘂𝗿𝗻 𝘀𝗮𝗺𝗲 𝗼𝘂𝘁𝗽𝘂𝘁𝘀

𝗘𝘅𝗮𝗺𝗽𝗹𝗲 👇 const add = (x, y) => x + y;

add(2, 4); // 6

So it will always 𝗿𝗲𝘁𝘂𝗿𝗻 𝘁𝗵𝗲 𝘀𝗮𝗺𝗲 𝗼𝘂𝘁𝗽𝘂𝘁 𝗶𝗳 𝘆𝗼𝘂 𝗽𝗮𝘀𝘀 𝟮 𝗮𝗻𝗱 𝟰.

Nothing else affects the output.

𝟭. 𝗡𝗼 𝘀𝗶𝗱𝗲-𝗲𝗳𝗳𝗲𝗰𝘁𝘀

The side effect is any interaction with the outside world from within a function. It should not produce any side effects like No mutating input, Http Calls, etc.

So the 𝗱𝗲𝗳𝗶𝗻𝗶𝘁𝗶𝗼𝗻 𝗼𝗳 𝗽𝘂𝗿𝗲 𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝘀 would be:

The function always returns the same result if the same arguments are pass. It does not depend on any state, or data, change during a program’s execution, and does not produce any observable side effects. It must only depend on its input arguments.

𝗘𝘅𝗮𝗺𝗽𝗹𝗲 👇

const add = (x, y) => x + y;

add(2, 4); // 6

// It will always return the same output if you pass 2 and 4