Why does converting a decimal number to an integer drop the fractional part?

Because some integer conversions truncate rather than round. Truncation removes the digits after the decimal point and moves toward zero: in JavaScript, Math.trunc(13.8) gives 13, not 14. Rounding is a separate rule: JavaScript Math.round(13.8) gives 14. [S1] [S2]

The word “convert” alone isn’t enough to identify the behavior. Check the specific function, operator, library, or input-reading method your code uses; different operations can have different rules. If you share the language and the exact expression, the behavior can be narrowed down.

Try a small diagnostic set and compare the outputs with the rules you intended. For example, with JavaScript’s Math.trunc(), 13.8 becomes 13 and −13.8 becomes −13: both move toward zero. With Math.round(), 13.8 becomes 14, while −13.8 becomes −14. The halfway cases also help identify the exact rule: JavaScript Math.round() sends 5.5 to 6 and −5.5 to −5, because exact halves go toward positive infinity. [S1] [S2]

Write down the actual operation alongside each result. A useful comparison is:

• Truncation: 13.8 → 13; −13.8 → −13.
• JavaScript Math.round(): 13.8 → 14; −13.8 → −14; 5.5 → 6; −5.5 → −5.

These examples describe those specific JavaScript methods, not every language’s conversion behavior. A different operation could use another rule.

That can mean the operation did not accept the value or input as supplied, rather than silently discarding its fractional part. The JavaScript references here explain Math.trunc() and Math.round(); they do not establish what every parser, type conversion, or language does with invalid or nonnumeric input. Don’t infer a general rejection rule from these examples.

To investigate, record the exact expression, the input’s type and displayed value, the output or full error message, and the language or runtime. Check the documentation for that precise operation, especially how it handles decimal values and invalid input. If you change the conversion method, test the same inputs again so you can see whether the new behavior matches your requirement.

Assuming your code is JavaScript and you want to make the choice explicit, call the method that matches your intended rule:

const value = 13.8;

const truncated = Math.trunc(value); // 13: removes the fractional part
const rounded = Math.round(value);   // 14: rounds to the nearest integer

Before relying on this in a larger calculation, check the rule for negative and halfway inputs too. For JavaScript, Math.trunc() moves toward zero, and Math.round() sends exact halves toward positive infinity. [S1] [S2] MDN also cautions that bitwise shortcuts are not interchangeable with Math.trunc() in all cases because they convert to 32-bit integers and can overflow. [S1]

Use the following checklist: name the operation; try positive and negative decimals; include a halfway value if rounding is intended; compare the result with the documented rule; and confirm how invalid input is handled. The cited material does not establish behavior for every language, conversion method, or input type, so check those separately.