While writing any post change mode from Visual to HTML and paste your javascript there.

Excluding this there are many plugins available which allows you to add javascript in WordPress posts.

You may try this plugin also Custom fields shortcode. Although this plugin hasn’t been updated in over 2 years but it works good without  showing any error in my blog.

Useage:

Use [cf] shortcode in your post content to show the custom field value without editing any of your WordPress theme files.

For Example:  [cf] FieldName[/cf] returns value of ‘FieldName’ custom-filed.

Check similar plugins here.