Trying to understand output from JavaScript eval which does String parsing

Question

Can anyone explain me why the following Java code which executes the JavaScript return "Smith" here?

    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("js");
    String jsSnippet = "var fullName = 'Joe Smith'; fullName.substring(fullName.indexOf(' ') + 1);";
    System.out.println(engine.eval(jsSnippet));

Similar JavaScript code (jsFiddle) which gives the same output as above Java code is

var jsSnippet = "var fullName = 'Joe Smith'; fullName.substring(fullName.indexOf(' ') + 1);";
document.write(eval(jsSnippet));

PS: I know eval is evil, but this is one of the circumstances I have to use JavaScript eval.

What exactly is the issue? A return value of Smith is what I would expect from the JS you have. — broofa
– broofa, Commented Jan 14, 2013 at 6:14
The question is unclear. You have 2 snippets of identical JS, and you say they both return the same value. What's the problem? — Alex Wayne
– Alex Wayne, Commented Jan 14, 2013 at 6:14
@broofa & Alex - There is no problem in the JS code, but I have not added any return statement in the eval, then why does the eval return the "Smith". And why it doesn't work if I add return statement in eval? jsfiddle.net/hs2501/xaJg4/2 — Hemant
– Hemant, Commented Jan 14, 2013 at 18:02

Evan Davis · Accepted Answer · 2013-01-14 21:19:20Z

2

The eval function evaluates the last expression passed into it, and returns its value. See the relevant section on eval at the Mozilla Developer Network.

edited Jan 14, 2013 at 21:19

Evan Davis

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answered Jan 14, 2013 at 6:17

Master_Yoda

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3 Comments

Hemant Over a year ago

I have not added any return statement in the eval, then why does the eval return the "Smith". And why it doesn't work if I add return statement in eval? jsfiddle.net/hs2501/xaJg4/2

Master_Yoda Over a year ago

Oh, the last expression is implicitly returned. Couldn't work out what you were asking, but there's your answer.

Master_Yoda Over a year ago

@Hemant, here's the relevant explanation of eval from the Mozilla Developer Network: developer.mozilla.org/en-US/docs/JavaScript/Reference/…

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