set-once

set-once allows you to define an object with certain properties that can be set once and only once. Once a property specified as set-once is set, subsequent attempts to set it will fail, either silently or with an exception (if the throwException option is specified).

Basic usage

An object is created by calling setOnce, passing an array of the names of properties to be treated as set-once. Other properties can be set more than once.

const setOnce = require('set-once');
 
const obj = setOnce(['name', 'email']);
obj.name = 'Joe'; 
obj.email = '[e-mail]';
obj.age = 38;
 
obj.name = 'Bob';
obj.email = '[e-mail]';
obj.age = 40;
 
console.log(obj.name);  // 'Joe' - second set has no effect
console.log(obj.email); // '[e-mail]' - second set has no effect
console.log(obj.age);   // 40 - non-set-once property can be updated

Sealing the object

To seal the returned object, pass the seal option. This will allow only the properties specified to be set. Attempts to add other properties will fail, either silently or with an exception (if in strict mode).

const setOnce = require('set-once');
 
const obj = setOnce(['name', 'email'], { seal: true });
obj.name = 'Joe';
obj.email = '[e-mail]';
obj.age = 38;
 
obj.name = 'Bob';
obj.email = '[e-mail]';
obj.age = 40;
 
console.log(obj.name);  // 'Joe'
console.log(obj.email); // '[e-mail]'
console.log(obj.age);   // undefined - object was sealed, only 'name' and 'email' properties are allowed

Throwing an exception

By default, attempting to set a set-once property a second time will fail silently. If you want an exception to be thrown in this case, pass the throwException option.

const setOnce = require('set-once');
 
const obj = setOnce(['name', 'email'], { throwException: true });
obj.name = 'Joe';
obj.email = '[e-mail]';
 
obj.name = 'Bob'; // exception is thrown here