Const Variable Modified in Cross Reference

Question

The code below declares an input_data as a const variable that is never modified in the lifetime of the execution. I expect every console.log to output the same thing 3 times but that is not the case.

This code is modified to be minimal and easily understandable and reflects the functions and promises am using in the real code. Could you tell me how and why this is happening and also show a working code with the same structure that doesn't modify that const variable

const platform = "test"; const input_data = { "test": true }; function verify(data) { let cmd = input_data; cmd.valid = true; return cmd; } console.log("#req", input_data); let command = verify(input_data); console.log("#req", input_data); new Promise((resolve, reject) => { command.executed = true; resolve("successs") }).then(result => { console.log("#req", input_data); });

Below is the output I get

#req { test: true } #req { test: true, valid: true } #req { test: true, valid: true, executed: true }

Instead it should be like this

#req { test: true } #req { test: true } #req { test: true }

javascript does assignment by reference. And declaring an object as const does not prevent you from changing the values at its keys. — The Fool
– The Fool, Commented Sep 23, 2020 at 8:51
yes you do. let cmd = input_data now command is your input object as it is a refence to the original. When you then do cmd.valid = true you added another key / value pair to the original object but you didnt violate the const. — The Fool
– The Fool, Commented Sep 23, 2020 at 8:53
because cmd is referencing input_data. I told you now three times. Please go learn about assignment by refrence. — The Fool
– The Fool, Commented Sep 23, 2020 at 8:56
if you want to prevent it, by the way, create a fresh brand new object. With es6 you could do let cmd = {...input_data, valid: true} — The Fool
– The Fool, Commented Sep 23, 2020 at 8:58

The Fool · Accepted Answer · 2020-09-23 09:27:55Z

You are doing assignment by reference.

When you do let cmd = input_data you reference the 'input_data' object. Meaning any change to cmd will reflect in input_data.

You can prevent this by creating a new object.

es5

let cmd = Object.assign({}, input_data, {valid: true})

es6

let cmd = {...input_data, valid: true}

So the final code would be

const platform = "test"; const input_data = {"test": true}; function verify(data){ let command = Object.assign({}, data); command.valid = true; return command; } console.log("#req", input_data); let command = verify(input_data); console.log("#req", input_data); new Promise( (resolve, reject) => { command.executed = true; resolve("successs") }).then( result => {console.log("#req", input_data);});

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