{"id":838,"date":"2019-04-05T10:37:00","date_gmt":"2019-04-05T10:37:00","guid":{"rendered":"https:\/\/staging.infragistics.com\/blogs\/?p=838"},"modified":"2025-02-26T08:43:19","modified_gmt":"2025-02-26T08:43:19","slug":"undeclared-variable-in-javascript","status":"publish","type":"post","link":"https:\/\/www.infragistics.com\/blogs\/undeclared-variable-in-javascript","title":{"rendered":"Type of Undeclared Variable in JavaScript: What is it?"},"content":{"rendered":"\n

Have you ever thought, what is type of undeclared variable<\/em> in JavaScript? I know, the first thing that might come to mind is: how can an undeclared variable have a type? Yes, in JavaScript it is possible.<\/p>\n\n\n\n

\"<\/figure>\n\n\n\n

To understand it, let us start with understanding types in JavaScript. There are seven built in types in JavaScript. They are as follows:<\/p>\n\n\n\n

    \n
  1. null<\/li>\n\n\n\n
  2. undefined<\/li>\n\n\n\n
  3. boolean<\/li>\n\n\n\n
  4. number<\/li>\n\n\n\n
  5. string<\/li>\n\n\n\n
  6. object<\/li>\n\n\n\n
  7. symbol (added on ES6)<\/li>\n<\/ol>\n\n\n\n
    \"<\/figure>\n\n\n\n

    Each variable with assigned value has a type. Let us consider the code listed below:<\/p>\n\n\n\n

    var\u00a0foo\u00a0=\u00a09;\nconsole.log(typeof\u00a0(foo));\u00a0\/\/\u00a0number\nvar\u00a0koo;\nconsole.log(typeof\u00a0(koo));\u00a0\/\/\u00a0undefined\nvar\u00a0too\u00a0=\u00a0'Infragistics';\nconsole.log(typeof\u00a0(too));\u00a0\/\/\u00a0string<\/pre>\n\n\n\n
    As you can see in the above snippet, if there is no value assigned then type of variable is undefined<\/strong>.  <\/p>\n\n\n\n
    \"<\/figure>\n\n\n\n
    So far so good, we saw that variable with no assigned value is undefined.  Let us consider the next code snippet:<\/p>\n\n\n\n
    var\u00a0koo;\nconsole.log(koo);\u00a0\/\/\u00a0undefiend\nconsole.log(typeof\u00a0(koo));\u00a0\/\/\u00a0undefined<\/pre>\n\n\n\n
    We have created a variable koo and have not assigned any value to it.  Now both value and type of koo are set to undefined.<\/p>\n\n\n\n
    \"<\/figure>\n\n\n\n
    Now that you understand type and value associated with undefined, let\u2019s move on to null. In JavaScript, null is a primitive type.  However, type of null value is object. Consider code listed below:<\/p>\n\n\n\n
    var\u00a0foo\u00a0=\u00a0null;\nconsole.log(foo);\u00a0\/\/\u00a0null\nconsole.log(typeof\u00a0(foo));\u00a0\/\/\u00a0object<\/pre>\n\n\n\n
    You may consider it as a legacy bug that type of null value is object in JavaScript.<\/p>\n\n\n\n
    \"<\/figure>\n\n\n\n
    Finally, yet equally as important, in JavaScript a variable which is not declared also has a type. The declared variable type is undefined.<\/p>\n\n\n\n
    console.log(foo);\/\/\u00a0error\u00a0foo\u00a0is\u00a0not\u00a0defined\nconsole.log(typeof\u00a0(foo));\u00a0\/\/\u00a0undefined\u00a0<\/pre>\n\n\n\n
    When you read the value of variable, which is  not declared, JavaScript will  return an error \u201cnot defined\u201d, and will return its type as undefined<\/p>\n\n\n\n
    \"<\/figure>\n\n\n\n
    In addition, keep in mind that in world of JavaScript not defined is not the same as undefined.  I hope now you understand various primitive types in JavaScript.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Have you ever thought, what is type of undeclared variable in JavaScript? I know, the first thing that might come to mind is: how can an undeclared variable have a type? Yes, in JavaScript it is possible.<\/p>\n","protected":false},"author":65,"featured_media":2210,"comment_status":"publish","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-838","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-how-to"],"_links":{"self":[{"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/posts\/838","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/users\/65"}],"replies":[{"embeddable":true,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/comments?post=838"}],"version-history":[{"count":1,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/posts\/838\/revisions"}],"predecessor-version":[{"id":2247,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/posts\/838\/revisions\/2247"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/media\/2210"}],"wp:attachment":[{"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/media?parent=838"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/categories?post=838"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.infragistics.com\/blogs\/wp-json\/wp\/v2\/tags?post=838"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}