Error when freeze and ignoreIfNoChange both true

[ariutta]

Should it be possible to set both freeze and ignoreIfNoChange to true?

When I run the following code using version 1.0.34:

var iassign = require("immutable-assign");

iassign.freeze = true;
iassign.ignoreIfNoChange = true;

var map1 = { a:1, b:2, c:3 };

// 1: Calling iassign() to update map1.b, using overload 2
var map2 = iassign(
    map1,
    function (m) { m.b = 50; return m; }
);

I get this error:

.../node_modules/immutable-assign/deploy/iassign.js:75
                newValue = setProp(obj);
                           ^
TypeError: Cannot assign to read only property 'b' of object '#<Object>'

[engineforce]

When ignoreIfNoChange is true, setProp() is called early with the input object (which is not the copy and is frozen), and its return value is compared with input object using ===. If no change occurred, iassign() can return early, improving performance.

There are two options to solve your problem:

1. Use getProp() to the extract the exact property you want to set (preferred):

var map2 = iassign(
    map1,
    function (m) { return m.b; },
    function (b) { return 50; }
);

2. Use Object.assign() to do the manual copy

var map2 = iassign(
    map1,
    function (m) { return Object.assign({}, m, { b: 50 }); }
);

You should use ignoreIfNoChange if you don't know if the setProp() will update your object or not, e.g., a reducer calling another reducer:

function AppReducer(state, action) {
    state = iassign(
        state,
        function(s) { return s.count; },
        function(c) { return CountReducer(c, action); },
        undefined,   // no context
        { ignoreIfNoChange: true });

    return state;
}

function CountReducer(count, action) {
    switch (action.type) {
    {
        case "INCREMENT": count++; break;
        case "DECREMENT": count--; break;
    }  
    return count;  // it will not change on other actions.
}

[ariutta]

That sounds reasonable. Thanks!