Node.js 101

1. Read-Eval-Print-Loop (REPL)

  • Node provides an interactive language shell, known as the Read-Eval-Print-Loop, or REPL.
  • REPL reads inputs from a user, evaluates the inputs as JavaScript codes, prints out the results, and then waits for more inputs.
  • REPL is useful for debugging and for experimenting with small snippets of JavaScript codes.
1) Starting the REPL and inputting some codes
  • The following shows how to start the REPL and input Javascript codes.
  • A variable, named foo, is created with the string value “Hello Node.js!”.
  • REPL prints “undefined” because the variable declaration statement returns no value.
  • Next, the statement foo; causes the value of foo to be inspected.
  • As expected, REPL returns the string “Hello Node.js!”.
  • Finally, the value of foo is printed to the terminal using the console.log() function.
  • After foo is printed, REPL displays “undefined” again, because console.log() returns no value.

2) Multiline lines of expression in REPL

3) REFL functions
  • Control + C: terminate any command and return to a blank prompt
  • Pressing Control+C twice from a blank line: REPL comes to end
  • Control + D: you can quit the REPL at any time.
  • Tab key: see a list of possible autocompletion optionss to the current command.
    • If there is only one possible option, Node automatically inserts it.
    • The list includes keywords, functions, and variables.

  • _ (underscore): it contains the result of the last expression.

4) REFL commands
  • .help: displays all of the available REPL commands
  • .exit: terminates the REPL
    • equivalent to pressing Control+D
  • .break: exits out of a multiline expression.
    • it is useful if you make a mistake or simply choose not to complete the expression
    • the normal > prompt is shown after the .break command.
  • .save filename: saves the current REPL session to the file specified in filename
  • .load filename: runs codes within the JavaScript file specified in filename
    • The codes are executed as if each line were typed directly into REPL.
  • .clear: resets the REPL’s context object.

2. Hello World

1) Just print out a string “Hello World”

2) Print “Hello” and then print “World” after two seconds.

3) Read a file ‘test.txt’ and print out the content

4) You can check that there is just one thread in node.js

Although an event occurs from ‘readFile’, the corresponding handler cannot get a chance to print the file’s content and the above program cannot terminate. The reason is why there is a thread in node.js.

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