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CHAPTER 4 DECISIONS & LOOPS
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A script can do different things depending on what values it has been passed.
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MAKING DECISIONS
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Is test score greater than 50?
Y Message: Try again… Message: You passed!
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if
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if (score > 50)
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Is test score greater than 50?
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if (score > 50) { document.write(‘You passed!’); }
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Is test score greater than 50?
Y
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Is test score greater than 50?
Y Message: You passed!
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if (score > 50) { document.write(‘You passed!’); } else { }
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Is test score greater than 50?
Y Message: You passed!
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if (score > 50) { document.write(‘You passed!’); } else { document.write(‘Try again…’); }
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Is test score greater than 50?
Y Message: Try again… Message: You passed!
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Is test score greater than 50?
Y Message: Try again… Message: You passed! CONTINUE SCRIPT…
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COMPARISON OPERATORS
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= = IS EQUAL TO
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= = IS EQUAL TO ! = IS NOT EQUAL TO
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= = IS EQUAL TO ! = IS NOT EQUAL TO = = = STRICT EQUAL TO
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= = IS EQUAL TO ! = = = = ! = = IS NOT EQUAL TO STRICT EQUAL TO
STRICT NOT EQUAL TO
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> GREATER THAN
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> GREATER THAN < LESS THAN
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> GREATER THAN < > =
LESS THAN > = GREATER THAN OR EQUAL TO
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> GREATER THAN < > = < =
LESS THAN > = GREATER THAN OR EQUAL TO < = LESS THAN OR EQUAL TO
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LOGICAL OPERATORS
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if (score > 75)&&(score < 95) { document.write(‘Very good!’); }
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if (score > 75)&&(score < 95) { document.write(‘Very good!’); }
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& & LOGICAL AND
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& & LOGICAL AND | | LOGICAL OR
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& & LOGICAL AND | | LOGICAL OR ! LOGICAL NOT
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SWITCH STATEMENTS
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switch
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switch (level) { }
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switch (level) { case ‘One’: title = ‘Level 1’; break; }
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switch (level) { case ‘One’: title = ‘Level 1’; break; case ‘Two’: title = ‘Level 2’; break; }
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switch (level) { case ‘One’: title = ‘Level 1’; break; case ‘Two’: title = ‘Level 2’; break; case ‘Three’: title = ‘Level 3’; break; }
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switch (level) { case ‘One’: title = ‘Level 1’; break; case ‘Two’: title = ‘Level 2’; break; case ‘Three’: title = ‘Level 3’; break; default: title = ‘Test’; break; }
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LOOPS
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for (var i = 0; i < 3; i++) { document.write(i); }
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for (var i = 0; i < 3; i++) { document.write(i); }
KEYWORD
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for (var i = 0; i < 3; i++) { document.write(i); }
CONDITION (COUNTER)
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for (var i = 0; i < 3; i++) { document.write(i); }
The variable i is declared and set a value of 0 INITIALIZATION
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for (var i = 0; i < 3; i++) { document.write(i); }
Every time the loop is run, the condition is checked to see if i is less than 3 CONDITION
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for (var i = 0; i < 3; i++) { document.write(i); }
If i is less than 3, the code block is run
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for (var i = 0; i < 3; i++) { document.write(i); }
The variable i can be used inside the loop (here it is used to write its value to the page)
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for (var i = 0; i < 3; i++) { document.write(i); }
When the code inside the curly braces has been executed, the variable i is incremented by 1 UPDATE
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ANATOMY OF A LOOP
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2 1 i = 0 i = 1 i = 2 i = 3
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i = 0
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Is i less than 3? i = 0
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Yes, i is less than 3 i = 0
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Write i to the page i = 0
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Add 1 to i i = 0 i = 1
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i = 0 i = 1
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i = 0 i = 1 Is i less than 3?
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i = 0 i = 1 Yes, i is less than 3
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i = 0 i = 1 Write i to the page 1
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i = 0 i = 1 Add 1 to i i = 2 1
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1 i = 0 i = 1 i = 2
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1 i = 0 i = 1 i = 2 Is i less than 3?
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1 i = 0 i = 1 i = 2 Yes, i is less than 3
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1 i = 0 i = 1 i = 2 Write i to the page 2
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1 i = 0 i = 1 i = 2 Add 1 to i i = 3 2
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2 1 i = 0 i = 1 i = 2 i = 3
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2 1 i = 0 i = 1 i = 2 i = 3 Is i less than 3?
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2 1 i = 0 i = 1 i = 2 i = 3 No, i is not less than 3
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2 1 i = 0 i = 1 i = 2 i = 3 End loop
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