--- title: 命令行解釋器 slug: Tools/Web_Console/The_command_line_interpreter translation_of: Tools/Web_Console/The_command_line_interpreter ---

You can interpret JavaScript expressions in real time using the command line provided by the Web Console.

Entering expressions

To enter expressions just type into the command line and press Enter. To enter multiline expressions, use Shift+Enter instead of Enter.

The expression you type is echoed in the message display window, followed by the result:

New in Firefox 47

From Firefox 47 onwards, if your input does not appear to be complete when you press Enter, then the Console treats this as Shift+Enter , enabling you to finish your input.

For example, if you type:

function foo() {

and then Enter, the Console will not immediately execute the input, but will behave as if you had pressed Shift+Enter , so you can finish entering the function definition.

Accessing variables

You can access variables defined in the page, both built-in variables like window and variables added by JavaScript like jQuery:

Autocomplete

The command line has autocomplete: enter the first few letters and a popup appears with possible completions:

Type Enter or Tab to accept the suggestion, use the up/down arrows to move to a different suggestion, or just keep typing if you don't like any of the suggestions.

The console suggests completions from the scope of the currently executing stack frame. This means that if you've hit a breakpoint in a function you get autocomplete for objects local to the function.

You get autocomplete suggestions for array elements, as well:

Defining variables

You can define your own variables, and then access them:

Command history

The command line remembers commands you've typed: to move back and forward through your history, use the up and down arrows.

From Firefox 39 onwards, this history is persisted across sessions. To clear the history, use the clearHistory() helper function.

Working with iframes

If a page contains embedded iframes, you can use the cd() command to change the console's scope to a specific iframe, and then you can execute functions defined in the document hosted by that iframe. There are three ways to select an iframe using cd():

You can pass the iframe DOM element:

var frame = document.getElementById("frame1");
cd(frame);

You can pass a CSS selector that matches the iframe:

cd("#frame1");

You can pass the iframe's global window object:

var frame = document.getElementById("frame1");
cd(frame.contentWindow);

To switch the context back to the top-level window, call cd() with no arguments:

cd();

For example, suppose we have a document that embeds an iframe:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
  </head>
  <body>
    <iframe id="frame1" src="static/frame/my-frame1.html"></iframe>
  </body>
</html>

The iframe defines a new function:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <script>
      function whoAreYou() {
        return "I'm frame1";
      }
   </script>
  </head>
  <body>
  </body>
</html>

You can switch context to the iframe like this:

cd("#frame1");

Now you'll see that the global window's document is the iframe: