In Odoo 15, you can call a field value in a JavaScript file by using the instance.web.form.FieldChar or instance.web.form.FieldSelection class, depending on the type of field you want to call.

For example, if you want to call the value of a Char field named "field_name", you can use the following code:

var field_value = instance.web.form.FieldChar.extend({
    init: function () {
        this._super.apply(this, arguments);
        this.field_value = this.get_value();
    },
});

And if you want to call the value of a Selection field named "field_name", you can use the following code

var field_value = instance.web.form.FieldSelection.extend({
    init: function () {
        this._super.apply(this, arguments);
        this.field_value = this.get_value();
    },
});


You can then use the field_value variable to access the value of the field in your JavaScript code.
It's also important to note that you will have to import the 'web.form_common' module first in your file.


odoo.define('module_name', function (require) {
    "use strict";
    var form_common = require('web.form_common');
    // your code here
}