What is the difference between the set and append methods of angulars HttpParams object?

HttpParams values are array of values.

When you set the value, it will override all the values in the array.

When you append the value, it will push new values on the existing array.

You can check the difference like this:

    let paramsSet = new HttpParams();
    paramsSet = paramsSet.set('paramName', 'set');

    let paramsAppend = new HttpParams();
    paramsAppend = paramsAppend.set('paramName', 'append');

    paramsSet = paramsSet.set('paramName', 'set2');
    paramsAppend = paramsAppend.append('paramName', 'append2');

    console.log(paramsSet.getAll('paramName'));
    console.log(paramsAppend.getAll('paramName')); 

paramsSet will only have ['set2'] as the value, while paramsAppend will have ["append", "append2"].

Set sets a unique value for the given key :

params.set('toto', '1').set('toto', '2') // toto=2

Append appends another value for the given key :

params.set('toto', '1').append('toto', '2') // toto=1&toto=2